dt-bindings: apmu: Document r8a7743 support

Document APMU and SMP enable method for RZ/G1M
(also known as r8a7743) SoC.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>