ARM: dts: renesas: Switch from ARCH_SHMOBILE_MULTI to ARCH_RENESAS

Commit 9b5ba0df ("ARM: shmobile: Introduce ARCH_RENESAS") started
the migration from ARCH_SHMOBILE_MULTI to ARCH_RENESAS.
Update the Makefile to build DTBs for Renesas platforms to use the new
symbol, and move the Renesas section to preserve sort order.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>