dt-bindings: apmu: Document r8a7745 support

Document APMU and SMP enable method for RZ/G1E (also known as
r8a7745) SoC.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>