dt-bindings: pinctrl: Convert brcm,iproc-gpio to DT schema

dt-bindings: pinctrl: Convert brcm,iproc-gpio to DT schema

Convert the Broadcom iProc/Cygnus GPIO/Pinconf binding to DT schema
format.

The child node structure is based on the example as there's not any
actual .dts files with child nodes.

The binding wasn't clear that "reg" can be 1 or 2 entries. The number of
"reg" entries doesn't appear to be based on compatible, so no per
compatible constraints for it

The "brcm,iproc-stingray-gpio" could possibly be dropped. There are no
.dts files using it, but the driver uses it.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/20250812203348.733749-1-robh@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>