soc/intel_adsp: Unify host window register interface

These registers are identical on all platforms, the only difference
being that cAVS 1.5 places them at a different address.

Create a devicetree node to track the register block, and replace the
platform header code with a global API defined once (it works like the
pre-existing shim struct).

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