linux-phytec-fsl

Documentation: bindings: document the sub-nodes AHCI bindings

The libahci now allows to use multiple PHYs and to represent each port
as a sub-node. Add these bindings to the documentation.

Signed-off-by: Antoine Ténart <antoine.tenart@free-electrons.com>
Signed-off-by: Tejun Heo <tj@kernel.org>