Sometimes a GPIO is needed to turn on/off the display.

Add support for this usecase by introducing the optional 'enable-gpios'
property.

Tested on a imx53qsb board.
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230314111724.1520178-2-festevam@denx.de