soc: intel_adsp: add a debug window slot descriptor

Add a debug window slot descriptor for the debug window area in the
first page, where descriptors themselves occupy the first few
hundreds of bytes. This area begins 1024 bytes after the beginning of
the debug window until the end of the page and is therefore 3 * 1024
bytes large. Descriptor 15 will be used to describe it and it will be
used for the GDB stub.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>