serial: s5p: get the port id number from the alias of the device node

The port id, if not specified in the device node, can be obtained from
the alias of the device node listed in the aliases node.

Cc: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>