arm64: dts: renesas: r9a09g047e57-smarc-som: Add PCIe reference clock

arm64: dts: renesas: r9a09g047e57-smarc-som: Add PCIe reference clock

The RZ/G3E SMARC SoM has a fixed 100 MHz reference clock generator
for PCIe.  Model it as a fixed-clock and assign it to the PCIe port.

Signed-off-by: John Madieu <john.madieu.xa@bp.renesas.com>
Tested-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com> # RZ/V2N EVK
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20260318085119.44717-4-john.madieu.xa@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>