linker: Create sections from zephyr,memory-region nodes

Currently when a node has a 'zephyr,memory-region' compatible and a
'zephyr,memory-region' string property, a new memory region is created
in the linker script.

Having a memory region without a section to place variables in could be
not that useful. With this patch we extend the memory-region mechanism
to also create sections.

The user can then place variables in the sections as usual by using for
example the GCC attributes.

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