dt-bindings: pinctrl: st,stm32: Document interrupt-controller property

dt-bindings: pinctrl: st,stm32: Document interrupt-controller property

Document interrupt-controller property and its interrupt-cells.
This fixes dtbs_check warnings when building current Linux DTs:

"
arch/arm/boot/dts/stm32mp153c-dhcom-drc02.dtb: pinctrl@50002000: gpio@5000a000: '#interrupt-cells', 'interrupt-controller' do not match any of the regexes: 'pinctrl-[0-9]+'
"

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220926204752.381798-1-marex@denx.de
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>