ARM: shmobile: r8a7779: Rely on DT for SMP CPU info

Remove r8a7779 specific ->smp_init_cpus() callback and
instead of relying on shmobile_smp_init_cpus() simply
use DT for CPU core information.

Based on work for the sh73a0 by Magnus Damm.

Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>