dt-bindings: gpio: describe Waveshare GPIO controller

dt-bindings: gpio: describe Waveshare GPIO controller

The Waveshare DSI TOUCH family of panels has separate on-board GPIO
controller, which controls power supplies to the panel and the touch
screen and provides reset pins for both the panel and the touchscreen.
Also it provides a simple PWM controller for panel backlight.

Add bindings for these GPIO controllers. As overall integration might be
not very obvious (and it differs significantly from the bindings used by
the original drivers), provide complete example with the on-board
regulators and the DSI panel.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://patch.msgid.link/20260507-waveshare-dsi-touch-v5-1-d2ac7ccc22d4@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>