PCI: endpoint: Add helper API to get the 'next' unreserved BAR

Add an API to get the next unreserved BAR starting from a given BAR number
that can be used by the endpoint function.

Link: https://lore.kernel.org/r/20210201195809.7342-4-kishon@ti.com


Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>