ARM: shmobile: r8a7791 SMP support

Tie in the APMU SMP code on r8a7791. When used together
with the secondary CPU device node and smp_ops in the
board specific code then this will allow use of the
two Cortex-A15 cores in the r8a7791 SoC.

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