[SERIAL] serial_txx9 driver update

 * Use platform_device.
 * Fix and cleanup suspend/resume/initialization codes.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>