[NONFUNC] Add documentation for stm32 driver

Documentation/devicetree/bindings/mfd/seco,stm32.txt

+Device tree bindings for the seco stm32 embedded controller
+***********************************************************
+
+The stm32 embedded controller acts as a GPIO- and PWM-controller
+and also handles system-reboot and -halt, by sending an appropriate
+command to the controller.
+
+Properties
+----------
+- compatible = "seco,stm32"
+- reg = <0x40>: The address on the i2c-bus.
+- pinctrl-names = "default"
+- pinctrl-0 = <phandle>: Phandle of the pincontrol, only required for the
+  interrupt-pin.
+- interrupt-parent = <phandle>: Where interrupts have to be send to.
+- interrupts = <phandle>: Number and flags of the interrupt-pin.
+
+GPIO
+- gpio-controller: This is a gpio-controller, all standard-properties of
+  a gpio-controller will automaticaly be available.
+- #gpio-cells = <2>: Number of parameters for a GPIO-definition,
+  pin-number and flags.
+- ngpios =/bits/ 16 <16-bit-int>: Number of GPIOs to handle.
+
+Interrupt
+- interrupt-controller: This is an interrupt-controller, all
+  standard-properties of an interrupt-controller will automaticaly be
+  available.
+- #interrupt-cells = <2>: Number of parameters for an
+  interrupt-definition, pin-number and flags.
+- cpu-irq = <int>: Interrupt that can be triggered by a GPIO.
+
+PWM
+- #pwm-cells = <2>: Number of parameters for a PWM-definition. These are
+  period and duty-cyle. This is only for the kernel. The periods of the
+  PWMs are hardcodes in the stm32-firmware and can not be changed.
+- pwm-pins = <array of int>: Pins that are used as PWM-pins.
+
+Reboot-notifier
+- reboot-delay-ms = <int>: Number of ms the embedded controller should
+  wait before actualy reboot of power-off.
+
+
+
+Examples
+--------
+&lpi2c5 {
+	stm32: stm32@40 {
+		reg = <0x40>;
+		compatible = "seco,stm32";
+		pinctrl-names = "default";
+		pinctrl-0 = <&pinctrl_stm32>;
+		interrupt-parent = <&gpio2>;
+		interrupts = <EC_IRQ_PIN IRQ_TYPE_EDGE_FALLING>;
+		gpio-controller;
+		#gpio-cells = <2>;
+		ngpios =/bits/ 16  <STM32_NGPIOS>;
+		interrupt-controller;
+		#interrupt-cells = <2>;
+		cpu-irq = <CPU_IRQ>;
+		#pwm-cells = <2>;
+		pwm-pins = <LCD0_BKLT_PWM_PIN LCD1_BKLT_PWM_PIN I2S0_MCLK_PIN>;
+		reboot-delay-ms = <500>;
+		status = "okay";
+	};
+};
+
+
+Using pin 10 as enable-gpio
+---------------------------
+some_node {
+	...
+	enable-gpio = <&stm32 10 GPIO_ACTIVE_HIGH>;
+	...
+};
+
+Using pin 11 as interrupt-source
+--------------------------------
+some_node {
+	...
+	interrupt-parent = <&stm32>;
+	interrupts = <11 IRQ_TYPE_EDGE_FALLING>;
+	...
+};
+
+Using pin 12 as PWM
+-------------------
+&stm32 {
+	pwm-pins = <12>; // pin 12 will act as pwm #0
+};
+
+some_node {
+	...
+	pwms = <&stm32 0 4000000 0>;
+	...
+};
+
+Using pin 13 as hog-gpio
+------------------------
+Sometimes you will need to switch a GPIO that is not handled by any driver,
+e.g. to enable an USB-hub or switch on a clock for a WiFi-interface.
+
+&stm32 {
+	some-gpio {
+		gpio-hog;
+		gpios = <13 (GPIO_PUSH_PULL | GPIO_ACTIVE_HIGH)>;
+		output-high:
+		line-name = "some-gpio";
+	};
+};
+
+
+
+Sysfs
+-----
+Some information is available in /sys/bus/i2c/devices/<bus>-<address>.
+These are:
+- chip_id
+- fw_ver
+- seco_code
+- boot_mode
+
+Writing to boot_mode will change the boot_mode for the next reboot.