Such property simply doesn't exist (is not documented or used anywhere).

This fixes:
arch/arm/boot/dts/bcm4708-asus-rt-ac56u.dtb: usb@21000: Unevaluated properties are not allowed ('#usb-cells' was unexpected)
        From schema: Documentation/devicetree/bindings/usb/generic-ehci.yaml
arch/arm/boot/dts/bcm4708-asus-rt-ac56u.dtb: usb@22000: Unevaluated properties are not allowed ('#usb-cells' was unexpected)
        From schema: Documentation/devicetree/bindings/usb/generic-ohci.yaml
arch/arm/boot/dts/bcm4708-asus-rt-ac56u.dtb: usb@23000: Unevaluated properties are not allowed ('#usb-cells' was unexpected)
        From schema: Documentation/devicetree/bindings/usb/generic-xhci.yaml
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20230602133455.7441-1-zajec5@gmail.comSigned-off-by: Florian Fainelli <florian.fainelli@broadcom.com>