MAINTAINERS: List Arm interconnect PMUs as supported

Whatever I may or may not have hoped for, looking after these drivers
seems to have firmly stuck as one of the responsibilities of the job Arm
pays me for, and I would still like to be aware of any other patches, so
make it official.

CC: Ilkka Koskinen <ilkka@os.amperecomputing.com>
CC: Jing Zhang <renyu.zj@linux.alibaba.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>