dt-bindings: ata: add ahci_imx electrical properties

Add the documentation for the electrical properties for the iMX SATA
controller.  There are many values for these, and listing them would
be error prone.  Refer readers to the device documentation and driver
source code for these details.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>
Reviewed-by: NShawn Guo <shawn.guo@freescale.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

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

@@ -13,6 +13,14 @@ Required properties:
   required entry in clock-names
 - clock-names : should include "sata", "sata_ref" and "ahb" entries
 
+Optional properties:
+- fsl,transmit-level-mV : transmit voltage level, in millivolts.
+- fsl,transmit-boost-mdB : transmit boost level, in milli-decibels
+- fsl,transmit-atten-16ths : transmit attenuation, in 16ths
+- 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.
+
 Examples:
 
 sata@02200000 {