[POWERPC] Fix typo: MMCR0_PMA0 != MMCR0_PMAO

pmc.c has:

#ifndef MMCR0_PMA0
#define MMCR0_PMA0     0

This one took a while to find.  Unfortunately its the wrong define
(number 0 vs letter O). Its probably worth removing this override, since
if our includes get screwed up we will have the same (hard to debug)
failure.

Fix it simply for now, so that we can backport to stable.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>