can: flexcan: add self wakeup support

If wakeup is enabled, enter stop mode, else enter disabled mode. Self wake
can only work on stop mode.

Starting from IMX6, the flexcan stop mode control bits is SoC specific,
move it out of IP driver and parse it from devicetree.

Signed-off-by: Aisheng Dong <aisheng.dong@nxp.com>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>