ARM: shmobile: Update r8a7779 to use scu_power_mode()

Update the SMP code for R8A7779 to make use of the
shared SCU function scu_power_mode() together with
the early setup code in shmobile_secondary_vector_scu.

With this patch in place the secondary CPUs modify the
SCU setting during early boot instead of letting other
CPUs deal with the coherency setting before boot. In
other words, we used to setup coherency before boot
in r8a7779_boot_secondary() but that bit is now instead
handled by the code in shmobile_secondary_vector_scu.

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