arm64: dts: renesas: r8a7796-salvator-x: Add SoC name to file header

Document clearly which SoC this DTS applies to, to distinguish from
Salvator-X boards equipped with other SoCs.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm64/boot/dts/renesas/r8a7796-salvator-x.dts

 /*
- * Device Tree Source for the Salvator-X board
+ * Device Tree Source for the Salvator-X board with R-Car M3-W
  *
  * Copyright (C) 2016 Renesas Electronics Corp.
  *