arm64: dts: renesas: salvator-xs: Factor out USB ch3 parts

Move the parts related to the fourth USB2 interface on Salvator-XS
boards to salvator-xs.dtsi, to 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 ES2.0+, its descriptions are
protected by the preprocessor symbol SOC_HAS_USB2_CH3, defined in
r8a77951.dtsi, and undefined in r8a77950.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/411df1adfd64ee82e5a0d1603df85d657fd4f8c0.1623087028.git.geert+renesas@glider.be