ARM: dts: imx6ul-kontron-bl-common: Fix RTS polarity for RS485 interface

The polarity of the DE signal of the transceiver is active-high for
sending. Therefore rs485-rts-active-low is wrong and needs to be
removed to make RS485 transmissions work.

Signed-off-by: Annette Kobou <annette.kobou@kontron.de>
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Fixes: 1ea4b76c ("ARM: dts: imx6ul-kontron-n6310: Add Kontron i.MX6UL N6310 SoM and boards")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>