PCI: dwc: Add callbacks for accessing dbi2 address space

Certain platforms like TI's AM654 do not have aseparate address space for
dbi2 instead they are accessed using the same address space as dbi
with some configuration bit set. In order to support such platforms,
add callbacks for accessing dbi2 address space.

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