can: flexcan: add support for DT property 'wakeup-source'

The flexcan controller can be forced as a wakeup source by
stating that explicitly in the device's .dts file using the
"wakeup-source" boolean property.

Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>