intel_idle: support Bay Trail

Bay Trail (BYT) is a family of Silvermont-core Atom Processor SOCs,
including the Intel Atom Processor Z36xxx and Z37xxx Series.

Although it shares the Silvermont core with Avoton,
BYT is optimized for mobile, and thus it supports
different power saving CPU idle states.

Note that not all versions of Bay Trail HW support all
of the states listed in the driver.

Signed-off-by: Len Brown <len.brown@intel.com>
Tested-by: Aubrey Li <aubrey.li@linux.intel.com>