dtb: santino: Increase the backlight PWM frequency
The initial display used for the port was the DataImage fg050720dsswdg01. It needs a lower backlight frequency of 200 Hz (or 5000000 ns cycle time). On most panels used by G&F we use 20 kHz as PWM frequency.
Set 20 kHz this the default again and overlay this value if necessary. Previously this information was part of the XML data, this is supposed to be imported to the device-tree nowadays.