arm64: dts: renesas: salvator-x(s): Factor out HDMI1 parts

Move the common parts related to the second HDMI 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 description are protected by
the preprocessor symbol SOC_HAS_HDMI1, 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/a1d9dcd481d36e5b6e78934e587f6391022030b5.1623087028.git.geert+renesas@glider.be