[POWERPC] 4xx: Implement udbg_getc() for 440

Implement udbg_getc() for 440, which fixes xmon input.

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>