dt-bindings: pci: Add DT binding for Toshiba Visconti PCIe controller

This commit adds the Device Tree binding documentation that allows
to describe the PCIe controller found in Toshiba Visconti SoCs.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Link: https://lore.kernel.org/r/20210723221421.113575-2-nobuhiro1.iwamatsu@toshiba.co.jp


[robh: reference snps,dw-pcie.yaml]
Signed-off-by: Rob Herring <robh@kernel.org>