devicetree: bindings: Renesas APMU and SMP Enable method

Add DT binding documentation for the APMU hardware and add "renesas,apmu"
to the list of enable methods for the ARM cpus.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>