PCI: Add helpers to request/release memory and I/O regions

Add helpers to request and release a device's memory or I/O regions.

With these helpers in place, one does not need to select a device's memory
or I/O regions with pci_select_bars() prior to requesting or releasing
them.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>