soc/intel_adsp: Remove platform.h

This header is now included in only one place, and just contains a
handful of very-bootloader-specific platform tunables that will never
be exported elsewhere.  Move that code into the C file.  Longer term
we should configure the memory controllers with devicetree as much as
practical, but there's no reason to keep this header around.

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