msm_serial_hs: Add support to get UART port reference using port index

Add support to get reference of UART port using its port index.
i.e. if you want reference of UART port for enumerated device as
/dev/ttyHS10, then use the exported API with argument as 10.

Change-Id: I45fc0f2e0d44b21653fd4c875fa3b2f5735fdf84
Signed-off-by: Mayank Rana <mrana@codeaurora.org>