tty: serial: msm_serial: document DT alias

Update devicetree binding for msm_serial to reflect msm_serial_probe()
getting line id (port number) from the serialN alias.

Signed-off-by: Frank Rowand <frank.rowand@sonymobile.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>