MAINTAINERS: update devicetree maintainers

As requested by Grant, adding myself as an additional devicetree maintainer.

Also, add Documentation/devicetree to the file list for devicetree.
Signed-off-by: NRob Herring <rob.herring@calxeda.com>
Acked-by: NGrant Likely <grant.likely@secretlab.ca>

MAINTAINERS

@@ -4728,10 +4728,12 @@ F:	drivers/i2c/busses/i2c-ocores.c
 
 OPEN FIRMWARE AND FLATTENED DEVICE TREE
 M:	Grant Likely <grant.likely@secretlab.ca>
+M:	Rob Herring <rob.herring@calxeda.com>
 L:	devicetree-discuss@lists.ozlabs.org (moderated for non-subscribers)
 W:	http://fdt.secretlab.ca
 T:	git git://git.secretlab.ca/git/linux-2.6.git
 S:	Maintained
+F:	Documentation/devicetree
 F:	drivers/of
 F:	include/linux/of*.h
 K:	of_get_property