arm64: dts: renesas: salvator-x(s): Factor out USB ch2 parts

Move the common parts related to the third USB2 interface on
Salvator-X(S) boards to salvator-common.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, its descriptions are protected by
the preprocessor symbol SOC_HAS_USB2_CH2, defined in r8a77951.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/ebc434502fc899b1f9d9640f560c3eaab6bb0927.1623087028.git.geert+renesas@glider.be