colibri_imx7: add Colibri iMX7S/iMX7D module support

This commit adds support for the Toradex Computer on Modules
Colibri iMX7S/iMX7D. The two modules/SoC's are very similar hence
can be easily supported by one board. The board code detects RAM
size at runtime which is one of the differences between the two
boards. The board also uses the UART's in DTE mode, hence making
use of the new DTE support via serial DM.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>