dts: bindings: Do not require 'interrupts' on STM32 GPIO nodes

I'm guessing STM32 GPIO nodes don't generate interrupts, because I can't
find any device tree nodes with 'compatible = "st,stm32-gpio"' and an
'interrupts' property.

Remove the required 'interrupts' property from the binding. This fixes a
bunch of errors in
https://github.com/zephyrproject-rtos/zephyr/issues/17532

.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>