tty: serial: msm: Add runtime PM and system sleep support

Add runtime pm and suspend/resume callback support to serial msm
driver so that clock resources are managed runtime to save power.

CRs-Fixed: 1061290
Change-Id: If2004dcf19649f68cc8fbe0506799d3c700293ff
Signed-off-by: Pramod Gurav <gpramod@codeaurora.org>
Signed-off-by: Runmin Wang <runminw@codeaurora.org>