misc: pci_endpoint_test: Add support for PCI_ENDPOINT_TEST regs to be mapped to any BAR

pci_endpoint_test driver assumes the PCI_ENDPOINT_TEST registers will
always be mapped to BAR_0. This need not always be the case like in TI's
K2G where BAR_0 is mapped to PCI controller application registers.

Add support so that PCI_ENDPOINT_TEST registers can be mapped to any BAR.
Change the bar_size used for BAR test accordingly.

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