PCI: endpoint: Add support to use _any_ BAR to map PCI_ENDPOINT_TEST regs

pci_epf_test always maps the PCI_ENDPOINT_TEST registers to BAR_0. But if
BAR_0 is reserved for some other purpose (like in TI's K2G BAR_0 is mapped
to application registers and cannot be used to map any other regions),
PCI_ENDPOINT_TEST registers cannot be mapped making pci_epf_test unusable.
Add support to use any BAR to map PCI_ENDPOINT_TEST registers.

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