msm_serial_hs: Add runtime PM support

Use Linux Runtime Power Management framework to
manage clock and other hardware resources.
This replaces the timer based implementation.
The runtime PM framework helps in making the power
management functionality of the driver cleaner and
more robust. Clients vote and unvote for resources
as needed.

Change-Id: Ie942d8a05d10479f42bdb4a7144add4d7dfda812
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>