arm64: dts: rockchip: Support poweroff on NanoPC-T6

The RK806 on the NanoPC-T6 can be used to power on/off the whole board.
Mark it as the system power controller.

Signed-off-by: Hugh Cole-Baker <sigmaris@gmail.com>
Link: https://lore.kernel.org/r/20231216212134.23314-1-sigmaris@gmail.com


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