The Designware root port config space is memory mapped accesses via the
DBI space by default. Add a common implementation
dw_pcie_own_conf_map_bus() for platforms to use.

Link: https://lore.kernel.org/r/20200821035420.380495-5-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>