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>