PCI: endpoint: Add helper to get first unreserved BAR

Add a helper function pci_epc_get_first_free_bar() to get the first
unreserved BAR that can be used for endpoint function.

Tested-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>