arm64: dts: renesas: rzg3s-smarc-som: Add PCIe reference clock

arm64: dts: renesas: rzg3s-smarc-som: Add PCIe reference clock

Versa3 clock generator available on RZ/G3S SMARC Module provides the
reference clock for SoC PCIe interface. Update the device tree to reflect
this connection.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Acked-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20251119143523.977085-5-claudiu.beznea.uj@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>