ntb: intel: Add Icelake (gen4) support for Intel NTB

Adding 4th generation Intel NTB support bits. There are a lot of common
parts that the gen4 NTB has with gen3 NTB on Skylake. The commonalities are
reused in gen4 Icelake NTB.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>