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>

MAINTAINERS

@@ -9461,6 +9461,7 @@ S:	Maintained
 F:	drivers/of/
 F:	include/linux/of*.h
 F:	scripts/dtc/
+F:	Documentation/ABI/testing/sysfs-firmware-ofw
 
 OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS
 M:	Rob Herring <robh+dt@kernel.org>