PCI: tegra: Implement ->{add,remove}_bus() callbacks

The configuration space mapping on Tegra is somewhat special, and in order
to avoid wasting virtual address space the configuration space for each bus
needs to be stitched together from several blocks which form a single
continuous virtual address range for accessors.

Currently the configuration space is mapped upon the first access to one of
its registers.  However, the mapping operation may sleep under certain
circumstances, so doing it from the configuration space accessors (they are
protected by a spin lock) will trigger a warning.

To avoid the warning, use the ->add_bus() callback to perform the mapping
at enumeration time when the operation is allowed to sleep.  Also add an
implementation of ->remove_bus() that undoes the mapping established by the
->add_bus() callback.  While it isn't currently possible to unload the
module, there is work underway to remedy this, and this code will come in
handy when that happens.

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