mfd: rk8xx: Add support for RK806 power off

Use DEV_OFF bit to power off the RK806 PMIC, when system-power-controller
is used in DTS.

Signed-off-by: Ondrej Jirman <megi@xff.cz>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20231019165732.3818789-5-megi@xff.cz


Signed-off-by: Lee Jones <lee@kernel.org>