dt-bindings: arm: rockchip: Add Anbernic RG353P and RG503

Add entry for the Anbernic RG353P and RG503 handheld devices.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220906210324.28986-3-macroalpha82@gmail.com


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