ARM: dts: imx: Add basic dtsi file for imx6sll

Add dtsi file for imx6sll.
Signed-off-by: NBai Ping <ping.bai@nxp.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Acked-by: NDong Aisheng <Aisheng.dong@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>