MN10300: Allow KGDB to use the MN10300 serial ports

Allow KGDB to use the MN10300 serial ports through the polled I/O interface
provided via the TTY/serial layer and the kgdboc driver.  This allows the
kernel to be started with something like:

	kgdboc=ttySM0,115200

added to the command line.

Signed-off-by: David Howells <dhowells@redhat.com>