arm: dts: imx7: add basic i.MX 7/Colibri iMX7 device tree

Add base device for NXP i.MX 7Solo/7Dual. The two SoC are very
similar and hence can share the same device tree for boot loaders
purpose.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>