mfd: rts5249: Add support for RTS5250S power saving

Enable power saving for RTS5250S as following steps:
1.Set 0xFE58 to enable clock power management.
2.Check cfg space whether support L1SS or not.
3.If support L1SS, set 0xFF03 to free clkreq.
4.When entering idle status, enable aspm
  and set parameters for L1SS and LTR.
5.Wnen entering run status, disable aspm
  and set parameters for L1SS and LTR.
If entering L1SS mode successfully,
electric current will be below 2mA.

Signed-off-by: Rui Feng <rui_feng@realsil.com.cn>
Signed-off-by: Lee Jones <lee.jones@linaro.org>