arm64: dts: rockchip: Add Powkiddy RGB20SX

arm64: dts: rockchip: Add Powkiddy RGB20SX

The Powkiddy RGB20SX is a portable game console extremely similar to
the existing RGB30 console. The key differences from the RGB30 are
as follows:

 - Realtek RTW8723DS WiFi and Bluetooth.
 - UART pads for debug console (UART2).
 - A function button (ADC channel 0).
 - A much larger battery (5000 mAh).

Otherwise, the device is identical to the RGB30, including the
hard-coded value on ADC channel 1 used to identify the device at
runtime.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Link: https://lore.kernel.org/r/20241001154016.87386-3-macroalpha82@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>