arm64: dts: r8a7795: Use renesas,rcar-gen3-usb2-phy fallback binding

A fallback binding for the Renesas R-Car Gen3 for USB2.0 PHY driver was
added by commit cde7bc36 ("phy: rcar-gen3-usb2: add fallback binding").
This patch makes use of this binding in the DT for the r8a7795 SoC.

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