msm_serial_hs_lite: Remove usage of PM Runtime for startup and shutdown

If UART application is running and system suspend happens, serial core
suspends the UART port by shuting down UART port. With that
msm_hsl_shutdown is calling pm_runtime_put_sync() from system suspend
context where before calling system suspend routine of UART driver,
it is already incremented UART device's PM refcount. Hence with that
pm_runtime_put_sync() from msm_hsl_shutdown() doesn't call UART driver's
PM runtime suspend routine. Hence UART device's Clock remains in ON state
which doesn't allow XO shutdown.

Serial core uses its own PM ops to enable/disable clock before invoking
any UART driver functionality. Hence remove usage of PM Runtime APIs
from startup() and shutdown() APIs of UART driver which makes sure that
suspend/resume is supported when UART application has opened the port
and not used any other way of preventing system suspend.

CRs-Fixed: 467750
Change-Id: Ie2b6e7eaeed518950674815c48a9ac3fe173d9e1
Signed-off-by: Mayank Rana <mrana@codeaurora.org>