arm64: dts: renesas: salvator-x(s): Factor out SATA parts

Move the common parts related to the SATA interface on Salvator-X(S)
boards to salvator-common.dtsi and salvator-xs.dtsi, to reduce current
and avoid future duplication of board descriptions.

As this interface is not present on all SoCs that can be found on
Salvator-X(S), but only on R-Car H3 and M3-N, its descriptions are
protected by the preprocessor symbol SOC_HAS_SATA, defined in
r8a77951.dtsi and r8a77965.dtsi.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/87f91290ec7b523aa508e920a169ecaddccc9144.1623087028.git.geert+renesas@glider.be