Spread-spectrum doesn't work with Cubox-i hardware, so we have to
disable this feature.  Add a DT property so that platforms can
indicate that this feature should not be enabled.

Having it as a negative property keeps existing DT files working.
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>