ARM i.MX5/6: Add dt support for generic boards

Apart from the iomux setup which may not be needed on custom
boards the kernel starts fine using the devicetree, so add
generic boards to the dt_compat entries.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>