arm64: dts: renesas: r8a77965: Add second port to rcar_sound placeholder

This node is just a placeholder but fills that function better if it does
not trigger build warnings. This update satisfies the requirement that
nodes with #address-cells/#size-cells properties have more than one child
node.

This is flagged by dtc as follows:
 # make dtbs W=1
 arch/arm64/boot/dts/renesas/r8a77965-salvator-x.dtb: Warning (graph_child_address): /soc/sound@ec500000/ports: graph node has single child node 'port@0', #address-cells/#size-cells are not necessary
 arch/arm64/boot/dts/renesas/r8a77965-salvator-xs.dtb: Warning (graph_child_address): /soc/sound@ec500000/ports: graph node has single child node 'port@0', #address-cells/#size-cells are not necessary

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