arm64: dts: rockchip: move Chromebook-specific Gru-parts to a separate file

Similar to rk3288-Veyron before, the Gru-series does contain Chromebook
(aka clamshell laptops) and non-Chromebook devices. And while the two
Chromebook devices Kevin and Bob are quite similar, Scarlet the tablet-
device is quite different in its design.

Therefore move the Chromebook parts into a gru-chromebook dtsi file
to make sharing easier.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>