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>

Documentation/devicetree/bindings/pinctrl/st,stm32-pinctrl.yaml

@@ -48,7 +48,8 @@ properties:
     description: Phandle+args to the syscon node which includes IRQ mux selection.
     $ref: /schemas/types.yaml#/definitions/phandle-array
     items:
-      - items:
+      - minItems: 2
+        items:
           - description: syscon node which includes IRQ mux selection
           - description: The offset of the IRQ mux selection register
           - description: The field mask of IRQ mux, needed if different of 0xf