isst-core.c should contain generic core APIs only.
Platform specific implementations/configurations should be removed from
this file.

Introduce isst-core-mbox.c and move all mbox/mmio specific functions to
this file.

Introduce struct isst_platform_ops which contains a series of callbacks
that used by the core APIs but need platform specific implementation.

No functional changes are expected.
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
[srinivas.pandruvada@linux.intel.com: changelog edits]
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>