ARM: imx: add initial imx6dl support

The i.MX6 DualLite/Solo is another i.MX6 family SoC, which is highly
compatible with i.MX6 Quad/Dual.  And that's why we choose to support
it using imx6q code with cpu_is_imx6dl() check when necessary.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>