regmap: Restore L: linux-kernel@vger.kernel.org entry

As with commit 981c3a4f (MAINTAINERS: Restore "L:
linux-kernel@vger.kernel.org" entries) restore the mailing list entry
for the regmap framework in order to assist users in finding the list
if they read the file instead of using get_maintainers.pl.

Signed-off-by: Mark Brown <broonie@linaro.org>