serial/pmac_zilog: Add console polling support

This implements poll_get_char and poll_put_char for console polling,
which enables kgdb to work on machines that use the pmac_zilog serial
driver, for example the Apple PowerMac G5.

Signed-off-by: Chris J Arges <arges@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>