arm64: dts: rockchip: Enable RTC support for Rock 5B

Add DT node to enable RTC support for Rock 5B board.

Signed-off-by: Shreeya Patel <shreeya.patel@collabora.com>
Reviewed-by: Christopher Obbard <chris.obbard@collabora.com>
Link: https://lore.kernel.org/r/20230405082711.46303-1-shreeya.patel@collabora.com


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