ARM: dts: r8a7791: Use renesas,rcar-gen2-usb-phy fallback binding

A fallback binding for the Renesas R-Car Gen2 PHY driver was
added by commit 7777cb8b ("phy: rcar-gen2: add fallback binding").
This patch makes use of this binding in the DT for the r8a7791 SoC.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>