ARM: dts: imx: Add basic dts support for imx6sll EVK board

Add dts file support for imx6sll EVK board.

Signed-off-by: Bai Ping <ping.bai@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Dong Aisheng <Aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>