edac: AMD8111 & AMD8131 Kconfig fixup

The amd8111_edac.c driver will fail allmodconfig on architectures other
than PPC, introduce Kconfig dependency to avoid this, since both AMD8111
and AMD8131 chips are only adopted on Maple so far.

Signed-off-by: Harry Ciao <qingtao.cao@windriver.com>
Cc: Doug Thompson <norsk5@yahoo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>