ASoC: topology: Move skl-tplg-interface.h to uapi

skl-tplg-interface.h describes firmware format details for Skylake
topology files. It is part of the ABI and should reside in the uapi
directory.

While moving the file, also replace the license boilerplate with
the SPDX License Identifier.

No functional change.

Signed-off-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>