arm64: dts: rockchip: add Gru Scarlet devicetrees

Gru-Scarlet is a tablet device using ChomeOS, dual-dsi display
and Wacom touchscreen with stylus.

There exist two variants in the market using different displays
that are differentiated via their sku-id.
The bootloader on them also determines the correct devicetree to
load via the sku-id.

So add a common scarlet dtsi and two minimal board devicetrees
for the two display variants.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Rob Herring <robh@kernel.org>