mfd: rk808: Add reboot support to rk808.c

This adds reboot support to the rk808 pmic driver and enables it for
the rk809 and rk817 devices.
This only enables if the rockchip,system-power-controller flag is set.

Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220208194023.929720-1-pgwipeout@gmail.com