MAINTAINERS: mmc: add maintainer entry for dw_mmc driver

Add maintainer entry for the Synopsys DW host driver which is used in
various SOC including EXYNOS series.  As Will Newton will no longer be
able to take care of dw_mmc*, I and Jaehoon Chung are willing to maintain
it.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>