power: pmic: rk8xx: Use sysreset implementation of the poweroff command

Select SYSRESET_CMD_POWEROFF to use the sysreset implementation of the
poweroff command when PMIC_RK8XX is enabled.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>