MAINTAINERS: add device tree ABI documentation file

sysfs-firmware-ofw describes the /sys/firmware/devicetree/ hierarchy
and /proc/device-tree so add a file line for it to the entry
OPEN FIRMWARE AND FLATTENED DEVICE TREE

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>