dt-bindings: usb: usb-xhci: add r8a77961 support

This patch adds support for r8a77961 (R-Car M3-W+).

To avoid confusion between R-Car M3-W (R8A77960) and R-Car M3-W+
(R8A77961), this patch also updates the comment of
"renesas,xhci-r8a7796".

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Felipe Balbi <balbi@kernel.org>