Enable i.MX6SLL SoC which is already supported by mainline kernel.

Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>