power: regulator: rk8xx: update the driver for rk808 and rk818

In order to adapt the following pmics, make the interface more compatible.
Support buck and ldo suspend voltage setting and getting.
Supprot buck and ldo suspend enable/disable setting and getting.

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>