Add Michael Ellerman as powerpc co-maintainer

Michael has been backing me up and helping will all aspects of
maintainership for a while now, let's make it official.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

MAINTAINERS

@@ -5357,6 +5357,7 @@ F:	arch/powerpc/boot/rs6000.h
 LINUX FOR POWERPC (32-BIT AND 64-BIT)
 M:	Benjamin Herrenschmidt <benh@kernel.crashing.org>
 M:	Paul Mackerras <paulus@samba.org>
+M:	Michael Ellerman <mpe@ellerman.id.au>
 W:	http://www.penguinppc.org/
 L:	linuxppc-dev@lists.ozlabs.org
 Q:	http://patchwork.ozlabs.org/project/linuxppc-dev/list/