arm/imx6: add imx6q sabrelite board support

- Add basic board dts file
- Add board compatible string to mach-imx6q.
- Update fsl DT board doc.

Signed-off-by: Richard Zhao <richard.zhao@linaro.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
3 files changed