dt-bindings: PCI: uniphier-ep: Add iATU register description

In the dt-bindings, "atu" reg-names is required to get the register space
for iATU in Synopsis DWC version 4.80 or later.

Link: https://lore.kernel.org/r/1601444167-11316-3-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>