arm64: dts: renesas: Extract common Salvator-X/XS board support

The Renesas Salvator-X and Salvator-XS (Salvator-X 2nd version) boards
are very similar.  To avoid duplication, prepare for the advent of the
latter by extracting the common board parts into its own .dtsi file.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>