board: rockchip: Add support for FriendlyARM NanoPi R2C Plus

The NanoPi R2C Plus is a small variant of NanoPi R2C with a on-board
eMMC flash (8G) included.

The device tree is taken from the kernel v6.5.

Signed-off-by: Tianling Shen <cnsztl@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>