soc/intel_adsp: Clean up host memory window layout

The locations of the memory windows within the reserved SRAM region
was being construted via a bunch of magic numbers referencing
(potentially historical?) SOF usage of those areas.  But Zephyr only
ever touches two of these windows, and only cares about the sizes and
offsets.  The complexity was hurting and not helping (especially since
there was no attempt made to unify these values with the ones that are
actually live in the SOF tree).

Replace with kconfig variables that simply specify the offset.  Only
one platform has a nonstandard layout anyway.  That allows SOF to move
things around in a clean way if it wants.  Ideally we should be
presenting a proper API for managing this region, though.

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