kgdb: kgdboc console poll hooks for serial_txx9 uart

Implement the serial polling hooks for the serial_txx9 uart for use with
kgdboc.

This patch once got SOB from Jason on Jul 2008 and (perhaps) merged into
kgdb-next branch, but lost somewhere then.  I resend it now with Jason's
Acked-by.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Acked-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>