PCI: tegra: Do not allocate MSI target memory

The PCI host bridge found on Tegra SoCs doesn't require the MSI target
address to be backed by physical system memory.  Writes are intercepted
within the controller and never make it to the memory pointed to.

Since no actual system memory is required, remove the allocation of a
single page and hardcode the MSI target address with a special address that
maps to the last 4 KiB page within the range that is reserved for system
memory and memory-mapped I/O in the FPCI address map.

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