PCI: vmd: Fix shadow offsets to reflect spec changes

The shadow offset scratchpad was moved to 0x2000-0x2010. Update the
location to get the correct shadow offset.

Fixes: 6788958e ("PCI: vmd: Assign membar addresses from shadow registers")
Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: stable@vger.kernel.org # v5.2+