dt-bindings: clock: renesas,r9a09g077/87: Add USB_CLK clock ID

Add the USB clock (USB_CLK) definition for the Renesas RZ/T2H
(R9A09G077) and RZ/N2H (R9A09G087) SoCs.  USB_CLK is used as the
reference clock for USB PHY layer.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/20250804202643.3967484-2-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>