dt-bindings: pinctrl: st,stm32: fix phandle-array warning

dt-bindings: pinctrl: st,stm32: fix phandle-array warning

make CHECK_DTBS=y st/stm32f469-disco.dtb

brings up a warning about a missing argument:

stm32f469-disco.dtb: pinctrl@40020000: st,syscfg:0: [21, 8] is too short

The description of the third entry indicates that this entry is optional.
The code in stm32_pctrl_dt_setup_irq parses st,syscfg and treats the third
entry as optional. It defaults to 0xf if not present in the devicetree.

Update the schema to require at least two entries, use the same syntax as
the description of renesas,ipmmu-main in
Documentation/devicetree/bindings/iommu/renesas,ipmmu-vmsa.yaml.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20231006082247.3830719-1-martin@kaiser.cx
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>