mx6memcal: enable SDP support

The initial implementation of mx6memcal reset the CPU after
running the memory calibration procedure because the generic
board has no information about which boot devices are available.

Now that we have SDP support in SPL, use it to allow a full
U-Boot to be uploaded (i.e. to use "mtest").

Signed-off-by: Eric Nelson <eric@nelint.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>