dt-bindings: arm: rockchip: add radxa rock3 model a

Add entry for the Radxa ROCK3 Model A board (with the Rockchip
RK3568 SoC).

Signed-off-by: Michael Riesch <michael.riesch@wolfvision.net>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220310210352.451136-2-michael.riesch@wolfvision.net


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