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 <[e-mail]>

MAINTAINERS

···
 LINUX FOR POWERPC (32-BIT AND 64-BIT)
 M:	Benjamin Herrenschmidt <[e-mail]>
 M:	Paul Mackerras <[e-mail]>
+M:	Michael Ellerman <[e-mail]>
 W:	http://www.penguinppc.org/
 L:	[e-mail]
 Q:	http://patchwork.ozlabs.org/project/linuxppc-dev/list/