PCI: dwc: Add common iATU register support

This gets iATU register area from reg property that has reg-names "atu".
In Synopsys DWC version 4.80 or later, since iATU register area is
separated from core register area, this area is necessary to get from
DT independently.

Suggested-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1601444167-11316-4-git-send-email-hayashi.kunihiko@socionext.com


Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Murali Karicheri <m-karicheri2@ti.com>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: Gustavo Pimentel <gustavo.pimentel@synopsys.com>