dts: bindings: Add memory-region property in bindings

Since the introduction of the `zephyr,memory-region` compatible
to create new memory region, drivers can now use the newly defined
region in the DT by using the macro LINKER_DT_NODE_REGION_NAME()
on the node_id to retrieve the region name and possibly allocated data
or variables in that region (or for whatever other use).
This is assuming though that the driver knows in advance the node_id to
reference.

A better approach is to add a new generic 'memory-region' property
that can be used by any driver to reference a region by using a phandle,
so that the driver can use that to retrieve the memory-region of interest.

Signed-off-by: Adam Wojasinski <adam.wojasinski@nordicsemi.no>
Signed-off-by: Carlo Caione <ccaione@baylibre.com>