ARM: dts: r8a7793: Add APMU node and second CPU core

Add DT nodes for the Advanced Power Management Unit (APMU) and the
second CPU core.  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>