msm: pm: Support for Krait Retention

Introduce a new low power state for Krait, retention.
This state will lower the voltage to the Krait core to
retention voltage in addition to clock gating. This state
will be in between WFI and Standalone power collapse in
terms of time overhead and power savings.

Change-Id: I6b1f0a0d2e4954a705a047fd4a07f4dd1fb3518a
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>
(cherry picked from commit d27ca4a7526a890f9d2979acefda3561e49d2e66)

Signed-off-by: Sudhir Sharma <sudsha@codeaurora.org>
(cherry picked from commit 3af7249c36ab53923fdf216df3b3ba2c7f847851)