Remove sh73a0 specific ->smp_init_cpus() callback and
instead of relying on shmobile_smp_init_cpus() simply
use DT for CPU core information.

This assumes that DT_MACHINE_START is used which is
the case when AG5EVM and Kota2 are removed.
Signed-off-by: NMagnus Damm <damm@opensource.se>
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>