PCI: vmd: Add offset to bus numbers if necessary

Depending on platform configuration, certain VMD devices may have an
additional configuration option which specifies the range of bus numbers
allowed in a VMD PCIe domain. We determine this requirement by checking
the value of two vendor specific config registers in the VMD endpoint:

 VMCAP[0] | VMCONFIG[9:8] | Bus Numbers
----------------------------------------
    0     |       *       |     0-255
    1     |      00       |     0-127
    1     |      01       |   128-255
    1     |      10       |     0-255

This feature is also added as a bit in driver_data, to allow future
conforming device ids which support these features to be enabled through
sysfs new_id.

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
[lorenzo.pieralisi@arm.com: updated commit subject]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>