mx7: Distinguish between dual and solo versions

Read the number of cores in the fuses to distinguish between
the dual and solo versions.

Tested on a mx7d sabresd and on a mx7solo warp7.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>