ARM: shmobile: sh73a0: Add Multiplatform support

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>