Currently in Intel SST driver, some similar IPC/mailbox processing
code are used in different platforms (e.g. in baytrail/broadwell).

This patch extracts the common code and creates new files
(sst-ipc.c/sst-ipc.h) to contain the common code and provide the generic
APIs for IPC/mailbox processing.
Signed-off-by: NJin Yao <yao.jin@linux.intel.com>
Acked-by: NJie Yang <yang.jie@intel.com>
Signed-off-by: NMark Brown <broonie@kernel.org>