PCI: cadence: Add support to configure virtual functions

Now that support for SR-IOV is added in PCIe endpoint core, add support
to configure virtual functions in the Cadence PCIe EP driver.

Link: https://lore.kernel.org/r/20210819123343.1951-7-kishon@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>