PCI: dwc: Cleanup DBI,ATU read and write APIs

Cleanup DBI read and write APIs by removing leading "__" (underscore)
from their names as there is no reason to have leading underscores
in the first place in the function definition.

Remove dbi/dbi2 base address parameters as the same behaviour can be
obtained through read and write APIs. Since dw_pcie_{readl/writel}_dbi()
APIs can't be used for ATU read/write as ATU base address could be
different from DBI base address, implement ATU read/write APIs using ATU
base address without using dw_pcie_{readl/writel}_dbi() APIs.

Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>