dt-bindings: gpio: mpfs-gpio: allow parsing of hog child nodes.

The SD card and eMMC on PolarFire SoC are sometimes muxed using a GPIO
by the bootloader. Add a hog child property to facilitate this.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>