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