PCI: tegra: Fix extended configuration space mapping

The 16 chunks of 64 KiB that need to be stitched together to make up the
configuration space for one bus (1 MiB) are located 24 bits (== 16 MiB)
apart in physical address space.  This is determined by the start of the
extended register field (bits 24-27) in the physical mapping.

Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Peter Daifuku <pdaifuku@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>