Documentation: bindings: Add the regulator property to the sub-nodes AHCI bindings

It is now possible to use a regulator property for each port of the
AHCI controller.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>