dt-bindings: pinctrl: rp1: Describe groups for RP1 pin controller

The DT binding for RP1 pin controller currently lacks the group
definitions.

Add groups enumeration to the schema.

Signed-off-by: Andrea della Porta <andrea.porta@suse.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/20250812084639.13442-1-andrea.porta@suse.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>