platform/x86: ISST: Add Intel Speed Select mailbox interface via MSRs

Add an IOCTL to send mailbox commands to PUNIT using PUNIT MSRs for
mailbox. Some CPU models don't have PCI device, so need to use MSRs.
A limited set of mailbox commands can be sent to PUNIT.

This MMIO interface is used by the intel-speed-select tool under
tools/x86/power to enumerate and control Intel Speed Select features.
The MBOX commands ids 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>