ARM: dts: r8a7745: Add APMU node and second CPU core

Add DT node for the Advanced Power Management Unit (APMU), add the
second CPU core, and use "renesas,apmu" as "enable-method".

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Chris Paterson <chris.paterson2@renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>