pci: pci-uclass: Add support for Single-Root I/O Virtualization

SR-IOV - Single Root I/O Virtualization
PF - Physical Function VF - Virtual Function

If SR-IOV capability is present, use it to initialize Virtual Function
PCI device instances. pci_sriov_init function will read SR-IOV
registers to create VF devices under the PF PCI device and also bind
driver if available. This function needs to be invoked from Physical
function device driver which expects VF device support, creating
minimal impact on existing framework.

Signed-off-by: Suneel Garapati <sgarapati@marvell.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>