linker: Introduce zephyr,memory-region compatible

Introduce a new "zephyr,memory-region" compatible to be used when a new
memory region must be created in the linker script from the devicetree
nodes using the compatible.

Remove also the LINKER_DT_REGION_FROM_NODE macro and add a new
LINKER_DT_REGIONS macro to cycle through all the compatible regions.

In the same PR modify the DTS files and the linker scripts.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>