arm64: dts: r8a7796: Add external PCIe bus clock

Add the external PCIe bus clock as a zero Hz fixed-frequency clock.
Boards that provide this clock should override it.

Based on r8a7795.dtsi.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>