The rcar_sound port nodes have unit names and thus should have register
properties.

This is flagged by dtc as follows:
 # make dtbs W=1
 ...
 DTC     arch/arm64/boot/dts/renesas/r8a77965-salvator-x.dtb
arch/arm64/boot/dts/renesas/r8a77965-salvator-x.dtb: Warning (unit_address_vs_reg): /soc/sound@ec500000/ports/port@0: node has a unit name, but no reg property
 DTC     arch/arm64/boot/dts/renesas/r8a77965-salvator-xs.dtb
arch/arm64/boot/dts/renesas/r8a77965-salvator-xs.dtb: Warning (unit_address_vs_reg): /soc/sound@ec500000/ports/port@0: node has a unit name, but no reg property

Prior to this patch the port nodes only defined in board DTS files.
As the register properties are common this patch defines the port nodes
and provides register properties in the SoC DTS file.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>