ARM: shmobile: armadillo800eva: Build DTS for multiplatform

Build the r8a7740 Armadillo800EVA board DTB in case
of Multiplatform. The DT reference case will be removed
and can be ignored for now.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>