Allow the rock-5b to poweroff its pmic. When issuing a "shutdown -h now"
on the rock-5b it reboots instead. Defining 'system-power-controller'
allows the rk806 to power down.

Commit c699fbfd ("arm64: dts: rockchip: Support poweroff on
NanoPC-T6") similarly resolves this issue for the nanopc-t6.
Signed-off-by: John Clark <inindev@gmail.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20231225222859.17153-1-inindev@gmail.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>