dt-bindings: PCI: pci-ep: Add binding to specify virtual function

Add binding to specify virtual function (associated with each physical
function) in endpoint mode.

Link: https://lore.kernel.org/r/20210819123343.1951-2-kishon@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>