ahci: st: Update the DT example for how to obtain the PHY.

The example is wrong in that the phys property should take a
phandle to the phy port.

Also with the changing over to generic PHY type constants we also
update that as well.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Tejun Heo <tj@kernel.org>