powerpc: Update ppc4xx maintainer

Alistair Popple has volunteered to take over maintainership of the ppc4xx
stuff upstream.  Switch the MAINTAINERS entry over to him.

Signed-off-by: Josh Boyer <jwboyer@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

MAINTAINERS

@@ -5216,11 +5216,10 @@ F:	arch/powerpc/platforms/512x/
 F:	arch/powerpc/platforms/52xx/
 
 LINUX FOR POWERPC EMBEDDED PPC4XX
-M:	Josh Boyer <jwboyer@gmail.com>
+M:  Alistair Popple <alistair@popple.id.au>
 M:	Matt Porter <mporter@kernel.crashing.org>
 W:	http://www.penguinppc.org/
 L:	linuxppc-dev@lists.ozlabs.org
-T:	git git://git.kernel.org/pub/scm/linux/kernel/git/jwboyer/powerpc-4xx.git
 S:	Maintained
 F:	arch/powerpc/platforms/40x/
 F:	arch/powerpc/platforms/44x/