Since there is already CONFIG_MX6S used for i.MX6 SOLO in u-boot codes,
we don't need to add new CONFIG_MX6SOLO. Rename the existing CONFIG_MX6SOLO
to CONFIG_MX6S.

Additional, for CONFIG_MX6S, we should select CONFIG_MX6DL. The major difference
for these two chips are core number and DDR controller. So all duallite
relevant definitions can apply to solo. User can combine the two configs
if any code only apply to solo or duallite.

Signed-off-by: Ye Li <ye.li@nxp.com>