Add driver for STM32
This is a complete rewrite of the driver in linux-5.15. It reflects all the changes in the STM32 firmware made for REVB0 of MAURY. This includes:
- IRQ-pins are no longer separated from GPIO-pins, instead the first 24 GPIO-pins can be used as IRQ-pins
- for each IRQ the flow-type can be set
- disabled (masked) IRQs are disabled in STM32 instead of simply been ignored
- several registers in the STM32-firmware has changed because enable- and status-registers have been added for the additional IRQs
- there is no default period for PWMs read from the device tree, this is up to the user of the PWM