dt-bindings: ata: document ability to disable spread-spectrum clock

Add documentation of the fsl,no-spread-spectrum option.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Reviewed-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/devicetree/bindings/ata/imx-sata.txt

@@ -20,6 +20,8 @@ Optional properties:
 - fsl,receive-eq-mdB : receive equalisation, in milli-decibels
   Please refer to the technical documentation or the driver source code
   for the list of legal values for these options.
+- fsl,no-spread-spectrum : disable spread-spectrum clocking on the SATA
+  link.
 
 Examples: