PCI: Add pcibios_sriov_enable() and pcibios_sriov_disable()

VFs are dynamically created when a driver enables them.  On some platforms,
like PowerNV, special resources are necessary to enable VFs.

Add platform hooks for enabling and disabling VFs.

Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>