MAINTAINERS: Add keyword match for of_match_table to device tree section

If a patch is working with an of_match_table it's probably adding an OF
binding to a driver in which case the binding ought to be reviewed by the
device tree folks to make sure that things like the device matches are set
up correctly.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>