arm64: dts: rockchip: make poweroff(8) work on Radxa ROCK 5A

Designate the RK806 PMIC on the Radxa ROCK 5A as the system power
controller, so the board shuts down properly on poweroff(8).

Fixes: 75fdcbc8

 ("arm64: dts: rockchip: add PMIC to rock-5a")
Reviewed-by: Dragan Simic <dsimic@manjaro.org>
Signed-off-by: FUKAUMI Naoki <naoki@radxa.com>
Link: https://lore.kernel.org/r/20240612033523.37166-1-naoki@radxa.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>