[POWERPC] Optionally use new device number for pmac_zilog

This adds the option for the pmac_zilog driver to use the major/minor
numbers recently allocated specifically for it (/dev/ttyPZn) instead
of the /dev/ttySn numbers.  The advantage of doing this is that it
allows the pmac_zilog and 8250 drivers to coexist.  The disadvantage
of doing this is that it is a user-visible ABI change and it will
break existing working setups on powermacs, and could be confusing to
users.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>