arm64: dts: meson-gxm: Add R-Box Pro

The R-Box Pro is a TV box derived from Amlogic q200 reference design.
It uses an AP6255 Wifi module. It features an LED tube that lights a
surrounding stripe and the top logo in blue or red or pink'ish - blue
is on by default, and red (i.e., pink) is configured as panic indicator.

This device is available in at least two models, with 2 GB vs. 3 GB RAM
as well as varying eMMC size. The intent is to handle this with a single
.dts that gets the actual RAM size from U-Boot.

Cc: ada@kingnoval.com
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>