pci: Add functions to read and write a BAR address

pci: Add functions to read and write a BAR address

Some PCI functions cannot be auto-configured. Add a function to set up a
fixed BAR which can be used in these situations. Also add a function to read
the current address of a BAR.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>