ARM: embestmx6boards: merge the riotboard's configs together

It doesn't make much sense to have two separate configs for
the riotboard so let's merge the SPL config into the main one
for less duplication.

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>