Add entries to the MAINTAINERS file for the new MPS2
board and devices.

Since the CMSDK devices are not specific to the MPS2 board,
extend the existing 'PrimeCell' section to cover CMSDK
devices as well; in both cases these are devices implemented
by ARM and provided as RTL that may be used in multiple
SoCs and boards.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1500029487-14822-10-git-send-email-peter.maydell@linaro.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>