MAINTAINERS: Add entry for Cirrus CODEC drivers

I noticed that some recent patches hadn't been CCed to the Cirrus people
- add an entry in MAINTAINERS to help make that happen.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Brian Austin <brian.austin@cirrus.com>
Acked-by: Paul Handrigan <Paul.Handrigan@cirrus.com>