intel_idle: initial IVB support

commit 6edab08c24f9141d69cfa4683a0a027d86ab303e upstream.

From an OS point of view, IVB looks like SNB, but quicker.

Signed-off-by: Len Brown <len.brown@intel.com>
[bwh: Backported to 3.2: add model number to the switch in
 intel_idle_probe(), as there is no device ID table]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>