PCI: Add pci_bus_address() to get bus address of a BAR

We store BAR information as a struct resource, which contains the CPU
address, not the bus address.  Drivers often need the bus address, and
there's currently no convenient way to get it, so they often read the
BAR directly, or use the resource address (which doesn't work if there's
any translation between CPU and bus addresses).

Add pci_bus_address() to make this convenient.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>