msm_serial: Cleanup regarding usage of clock enable/disable apis

Currently driver uses clock apis (clock_enable()/clock_disable()) before
accessing any UART Core register. Hence remove all usage of clock apis
which is just increasing the refcount of uart clock instead of turning
on/off the uart lock. Start using serial core PM functionality to enable
disable the uart clock.

Change-Id: I42e180fe6b3b92e4d9cc6d289f5f4689658e51c6
Signed-off-by: Mayank Rana <mrana@codeaurora.org>