dt-bindings: usb: renesas,usb-xhci: convert to YAML

Convert Renesas related parts of usb-xhci to YAML because
they are not compatible with generic-xhci.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/1592822252-12338-1-git-send-email-yoshihiro.shimoda.uh@renesas.com


Signed-off-by: Rob Herring <robh@kernel.org>