arm64: dts: rockchip: fix poweroff on helios64

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

Fixes: 09e006cf

 ("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>