ARM: dts: r8a7792: add SMP support

Add the device tree nodes for the Advanced Power Management Unit (APMU)
and the second Cortex-A15 CPU core.
Use the "enable-method" prop  to point out that the APMU should be used
for the SMP support.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>