dt-bindings: pinctrl: Convert brcm,bcm2835-gpio to DT schema

dt-bindings: pinctrl: Convert brcm,bcm2835-gpio to DT schema

Convert the Broadcom BCM2835 GPIO (and pinmux) controller binding to DT
schema format.

The structure of the child nodes wasn't well defined. The schema is
based on the .dts users. The legacy binding is a single level of child
nodes while the standard binding is 2 levels of child nodes.

The "all banks" interrupt is treated as optional following actual users.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/20250812203337.731648-1-robh@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>