thunderbolt: Separate out common NHI bits

thunderbolt: Separate out common NHI bits

Add a new file encapsulating most of the PCI NHI specifics
(intentionally leaving some odd cookies behind to make the layering
simpler). Most notably, separate out nhi_probe() to make it easier to
register other types of NHIs.

Also, fold in Intel Icelake (nhi_ops.c) support to contain all
PCIe-related bits in pci.c.

Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>