dt-bindings: ata: create bindings for imx sata controller

To: Tejun Heo <tj@kernel.org>,linux-ide@vger.kernel.org

The Freescale i.MX SATA controller mostly conforms to the AHCI
interface, but there are some special extensions at integration level
like clocks settings and hardware parameters.

Let's create a separate bindings doc for imx sata controller, so that
more imx specific properties can be added later without messing up the
generic ahci-platform bindings.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Tejun Heo <tj@kernel.org>