MAINTAINERS: Update ARM SCMI entry

Cristian is actively developing new features and more involved than me.
So add Cristian as a designated reviewer. Also add the newly added scmi
regulator driver to the list.

Link: https://lore.kernel.org/r/20210105151945.406093-1-sudeep.holla@arm.com


Cc: Cristian Marussi <cristian.marussi@arm.com>
Acked-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>