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: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alexander Graf <agraf@csgraf.de>
Acked-by: David Gibson <david@gibson.dropbear.id.au>