dt-bindings: phy: renesas,usb2-phy: Add clock constraint for RZ/G2L family

The RZ/G2L family requires two clocks for USB2 PHY, which are already
defined in the DTSI files. Add a constraint in the DT binding document
to ensure validation with `dtbs_check`.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250414145729.343133-2-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>