[POWERPC] Update PowerPC 4xx entry in MAINTAINERS

Add myself as PowerPC 4xx maintainer and list the git tree
Signed-off-by: NJosh Boyer <jwboyer@linux.vnet.ibm..com>

MAINTAINERS

@@ -2295,10 +2295,13 @@ L:	linuxppc-embedded@ozlabs.org
 S:	Maintained
 
 LINUX FOR POWERPC EMBEDDED PPC4XX
+P:	Josh Boyer
+M:	jwboyer@linux.vnet.ibm.com
 P:	Matt Porter
 M:	mporter@kernel.crashing.org
 W:	http://www.penguinppc.org/
 L:	linuxppc-embedded@ozlabs.org
+T:	git kernel.org:/pub/scm/linux/kernel/git/jwboyer/powerpc.git
 S:	Maintained
 
 LINUX FOR POWERPC BOOT CODE