msm : Add support to get user-defined TTY device for uart port

Currently UART port is enumerating with platform device ID. It is not
necessary that TTY<ID> is same as available UART device ID. Provide
optional support which allows to override used platform device ID with
uart port by userid to get TTY based Uart device as /dev/ttyMSM<userid>
instead of /dev/ttyMSM<ID>.This support helps third party application/test
framework to use the same TTY uart device name across different platform
without changing in those applications as it is not necessary that on
different platform used UART device would be having same TTY<ID> name.
The userid value should be from 0 to 256

Change-Id: If24ab20f0e4c5bb947b947ad0811f5a56a2c45db
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>