ahci: st: Provide DT bindings for ST's SATA implementation

Cc: devicetree@vger.kernel.org
Cc: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Acked-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Tejun Heo <tj@kernel.org>