overlays: add hdmi-backlight-hwhack-gpio-overlay

This is a Devicetree overlay for GPIO based backlight on/off capability.

Use this if you have one of those HDMI displays whose backlight cannot be controlled via DPMS over HDMI and plan to do a little soldering to use an RPi gpio pin for on/off switching.

See: https://www.waveshare.com/wiki/7inch_HDMI_LCD_(C)#Backlight_Control



This was tested with a clone of the Waveshare "7 inch HDMI Touch LCD C" where I soldered two mosfets to override the backlight dip-switch.
When the overlay is loaded, a sysfs backlight node appears which can be used to modify the brightness value (0 or 1), and is even used by DPMS to switch the display backlight off after the configured timeout.
(On current Raspbian Buster Desktop, it's also possible to wakeup the display via a tap on the touch display :-) )

Signed-off-by: Michael Kaplan <m.kaplan@evva.com>