docs(maintainers): add Chris Kay to AMU and MPMM
docs(maintainers): add Chris Kay to AMU and MPMM

Change-Id: I8c775c8cac4fbbb2904952747a9572a71aff37b4
Signed-off-by: Chris Kay <9c0c41cd71461f9de7536bb7a214db3cff1e29f1@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware