arm64: dts: renesas: Sort DTBs in Makefile

Sort the entries for the various DTBs in the Makefile by SoC and board
type.  Keep Salvator-X(S) together, and do the same for ULCB with and
without Kingfisher extension.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20191217183841.432-4-geert+renesas@glider.be