ARM: dts: imx6ul: Add flexcan stop mode wakeup support

Add stop-mode property which is required by stop mode wakeup
feature.

Signed-off-by: Aisheng Dong <aisheng.dong@nxp.com>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>