board: colibri_imx7: reserve DDR memory for Cortex-M4

i.MX 7's Cortex-M4 core can run from DDR and uses DDR memory for
the rpmsg communication. Both use cases need a fixed location of
memory reserved. For the rpmsg use case the reserved area needs
to be in sync with the kernel's hardcoded vring descriptor location.

Use the linux,usable-memory property to carve out 1MB of memory
in case the M4 core is running. Also make sure that the i.MX 7
specific rpmsg driver does not get loaded in case we do not carve
out memory.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>