ARM: embestmx6boards: convert the mars/riot boards to DM SPI

Enable DM_SPI and DM_SPI_FLASH on the mars/riot boards.

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>