Documentation: dt-bindings: Add a new compatible for the Armada 3700

The AHCI interfaces used in the Armada 3700 has a few differences with
the one used in the Armada 38x, so it deserves its own compatible string.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>