dt-binding: spi: add NPCM PSPI controller documentation

Added device tree binding documentation for Nuvoton BMC
NPCM Peripheral SPI controller.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/nuvoton,npcm-pspi.txt

+Nuvoton NPCM Peripheral Serial Peripheral Interface(PSPI) controller driver
+
+Nuvoton NPCM7xx SOC support two PSPI channels.
+
+Required properties:
+ - compatible : "nuvoton,npcm750-pspi" for NPCM7XX BMC
+ - #address-cells : should be 1. see spi-bus.txt
+ - #size-cells : should be 0. see spi-bus.txt
+ - specifies physical base address and size of the register.
+ - interrupts : contain PSPI interrupt.
+ - clocks : phandle of PSPI reference clock.
+ - clock-names: Should be "clk_apb5".
+ - pinctrl-names : a pinctrl state named "default" must be defined.
+ - pinctrl-0 : phandle referencing pin configuration of the device.
+ - cs-gpios: Specifies the gpio pins to be used for chipselects.
+            See: Documentation/devicetree/bindings/spi/spi-bus.txt
+
+Optional properties:
+- clock-frequency : Input clock frequency to the PSPI block in Hz.
+		    Default is 25000000 Hz.
+
+Example:
+
+spi0: spi@f0200000 {
+	compatible = "nuvoton,npcm750-pspi";
+	reg = <0xf0200000 0x1000>;
+	pinctrl-names = "default";
+	pinctrl-0 = <&pspi1_pins>;
+	#address-cells = <1>;
+	#size-cells = <0>;
+	interrupts = <GIC_SPI 31 IRQ_TYPE_LEVEL_HIGH>;
+	clocks = <&clk NPCM7XX_CLK_APB5>;
+	clock-names = "clk_apb5";
+	cs-gpios = <&gpio6 11 GPIO_ACTIVE_LOW>;
+};