ARM: embestmx6boards: convert mars/riot boards to DM_USB

Convert the marsboard/riotboard to use DM_USB.

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>