MAINTAINERS: Add entry for Cavium MMC driver

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Steven J. Hill <steven.hill@cavium.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>