ARM: dts: r8a7794: Add SMP support

Add the device tree node for the Advanced Power Management Unit (APMU).
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>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>