arm64: dts: renesas: Restructure Makefile

Make the Makefile for building Renesas DTB files easier to read and
maintain:
  - Get rid of line continuations,
  - Use a single entry per line,
  - Sort SoCs and boards alphabetically,
  - Separate SoCs by blank lines.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200706154015.29257-1-geert+renesas@glider.be