ARM: embestmx6boards: convert the mars/riot boards to DM_MMC

Convert the two Embest boards to use DM MMC.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Cc: "Eric Bénard" <eric@eukrea.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>