MAINTAINERS: Update the device tree maintainers

Remove Alex as a Device Tree maintainer as requested by him. Add myself
as a maintainer to avoid it being orphaned. Also add David as a
Reviewer (R) as he is the libfdt and DTC maintainer.
Signed-off-by: NAlistair Francis <alistair.francis@wdc.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NAlexander Graf <agraf@csgraf.de>
Acked-by: NDavid Gibson <david@gibson.dropbear.id.au>

MAINTAINERS

@@ -1813,7 +1813,8 @@ F: qom/cpu.c
 F: include/qom/cpu.h
 
 Device Tree
-M: Alexander Graf <agraf@suse.de>
+M: Alistair Francis <alistair.francis@wdc.com>
+R: David Gibson <david@gibson.dropbear.id.au>
 S: Maintained
 F: device_tree.c
 F: include/sysemu/device_tree.h