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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

MAINTAINERS

···
 F:	drivers/regulator/
 F:	include/dt-bindings/regulator/
 F:	include/linux/regulator/
+K:	regulator_get_optional
 
 VRF
 M:	David Ahern <[e-mail]>