MAINTAINERS: Add DT bindings to regulator framework entry

The regulators DT bindings docs and shared headers used by regulator
drivers and DTS are not listed as files for the regulator subsystem.

So developers may not know who should receive patches to these dirs
unless they rely on the get_maintainer.pl git-fallback option which
usually makes more harm than good.

This patch makes the correct recipient to be obtained using commands
such as scripts/get_maintainer.pl --no-git-fallback.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>