ARM: shmobile: kzm9g: Build DTS for Multiplatform

Build the sh73a0 KZM9G board DTB in case of Multiplatform.
The DT reference case will be removed in the future and
can be ignored for now.

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