arm64: dts: rockchip: fix poweroff on helios64

arm64: dts: rockchip: fix poweroff on helios64

[ Upstream commit 0d0cd1aeb18a67479291e417fb2702cd47b3f61c ]

Adding the rockchip,system-power-controller property here will use the
rk808 to power off the system.

Fixes: 5c8cf7f43e2a ("arm64: dts: rockchip: Add basic support for Kobol's Helios64")
Signed-off-by: Florian Klink <flokli@flokli.de>
Tested-by: Dennis Gilmore <dgilmore@redhat.com>
Link: https://lore.kernel.org/r/20211020095926.735938-2-flokli@flokli.de
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>