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