MAINTAINER, git-mailrc: Update the mmc maintainer

Update to me as co-maintainer with Peng.
Additionally, update the mmc alias in git-mailrc.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Peng Fan <peng.fan@nxp.com>