ARM: shmobile: smp: Add function to prioritize DT SMP

Add a function to check if other DT based method is available, and
if so return false to not hook up smp_ops from the machine vector.

This results in that DT-based SMP support has priority over older
C-based smp_ops code, and in case DT-based SMP support code does not
exist in the DTB then the old smp_ops code will still work as-is.

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>