mx6cuboxi: Remove obsolete comment

We currently support various versions of Solidrun mx6 boards, so
remove the obsolete comment.

Reported-by: Jon Nettleton <jon.nettleton@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>