devicetree_regions: Remove path fallback and sanitize name

This patch is doing two things:

- it is removing the fallback on the path. This is not possible anymore
  since the DT binding file is now actually requiring the
  'zephyr,memory-region' property to be present from which the region
  name is obtained.

- it is sanitizing the name when CONFIG_CMAKE_LINKER_GENERATOR is used
  or not.

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