arm64: dts: renesas: r8a77965: Add #phy-cells property

Add "#phy-cells" property to "usb-phy@e65ee000" device node.

This silences the following DTC compiler warning:
Warning (phys_property): Missing property '#phy-cells' in node
/soc/usb-phy@e65ee000 or bad phandle (referred from
/soc/usb@ee020000:phys[0])
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm64/boot/dts/renesas/r8a77965.dtsi

@@ -795,6 +795,7 @@ sdhi3: sd@ee160000 {
 
 		usb3_phy0: usb-phy@e65ee000 {
 			reg = <0 0xe65ee000 0 0x90>;
+			#phy-cells = <0>;
 			/* placeholder */
 		};