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>

MAINTAINERS

@@ -7550,6 +7550,7 @@ F:	fs/reiserfs/
 
 REGISTER MAP ABSTRACTION
 M:	Mark Brown <broonie@kernel.org>
+L:	linux-kernel@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git
 S:	Supported
 F:	drivers/base/regmap/