gpio: add GPIO controller found on Waveshare DSI TOUCH panels

gpio: add GPIO controller found on Waveshare DSI TOUCH panels

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
support for this GPIO controller.

Tested-by: Riccardo Mereu <r.mereu@arduino.cc>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://patch.msgid.link/20260507-waveshare-dsi-touch-v5-2-d2ac7ccc22d4@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>