soc/intel_adsp: Unify "RAM" region

There was a divergent set of definitions for a "RAM" region for the
benefit of memory layout in the platform headers.  In fact there was
only one platform dependence (cAVS 1.5 has 32k instead of 64k
reserved).  Put that into kconfig in a single place, and add a warning
that this is a trap region with hidden dependencies in both Zephyr and
SOF.  Good enough until we clean this up and make everything visible
to the linker.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>