linker: add an initialized DATA_SECTIONS linker location option

Current location options for linker source files includes init and
noinit ram data, but only a noinit ram section. This makes it impossible
for application code to define an initialized RAM output section,
such as with the Z_ITERABLE_SECTION_RAM() helpers.

Adding a DATA_SECTIONS linker source option for this use case.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>