dt-bindings: phy: renesas: usb2-phy: add r8a77961 support

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

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/1585301636-24399-3-git-send-email-yoshihiro.shimoda.uh@renesas.com
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>