MAINTAINERS: add mbolivar for devicetree

I was a devicetree maintainer for several years, but had to step away
from the role for various reasons. I've received approval to resume
this work at my current employer. Add myself to the maintainers list
and restore the 'maintained' status of the subsystem.

Signed-off-by: Martí Bolívar <marti.bolivar@oss.qualcomm.com>