MAINTAINERS: add regulator.rs to the regulator API entry

Add this file to the regulator API entry as requested by Mark Brown.

Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://patch.msgid.link/20250714-topics-tyr-regulator2-v8-2-c7ab3955d524@collabora.com


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