MAINTAINERS: Add keyword pattern on regulator_get_optional()

In an effort to try to contain abuses of regulator_get_optional() add a
keyword entry to the MAINTAINERS stanza for the regulator API so that the
regulator maintainers get CCed on new usages.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20190829125435.48770-1-broonie@kernel.org


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