platform/x86: ISST: Add Intel Speed Select mmio interface

Added MMIO interface to read/write specific offsets in PUNIT PCI device
which export core priortization. This MMIO interface can be used using
ioctl interface on /dev/isst_interface using IOCTL ISST_IF_IO_CMD.

This MMIO interface is used by the intel-speed-select tool under
tools/x86/power to enumerate and set core priority. The MMIO offsets and
semantics of the message can be checked from the source code of the tool.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>