Enable sh73a0 Multiplatform support for the generic sh73a0
machine vector. No board support is enabled, and the board
code for KZM9G DT Reference is left by itself.

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>