dt-bindings: arm: bcm: raspberrypi,bcm2835-firmware: Add gpio child node

dt-bindings: arm: bcm: raspberrypi,bcm2835-firmware: Add gpio child node

Unlike the other child nodes of the raspberrypi,bcm2835-firmware device,
the gpio child is documented in a legacy text-based binding in
gpio/raspberrypi,firmware-gpio.txt. This causes DT validation failures:

arch/arm64/boot/dts/broadcom/bcm2711-rpi-4-b.dtb: 'gpio' does not match any of the regexes: 'pinctrl-[0-9]+'
        from schema $id: http://devicetree.org/schemas/arm/bcm/raspberrypi,bcm2835-firmware.yaml#

Convert the binding to YAML and move it to
raspberrypi,bcm2835-firmware.yaml.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Stefan Wahren <wahrenst@gmx.net>
Tested-by: Ivan T. Ivanov <iivanov@suse.de>
Link: https://lore.kernel.org/r/20240326195807.15163-2-laurent.pinchart@ideasonboard.com
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>