ARM: dts: r8a7791: Add APMU node

Add a DT node for the Advanced Power Management Units (APMU), and use
the enable-method to point out that the APMU should be used for SMP
support.

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