dt-bindings: can: tcan4x5x: Document the wakeup-source flag

Let it be known that the tcan4x5x device can now be configured to wake
the host from suspend when a can frame is received.

Signed-off-by: Martin Hundebøll <martin@geanix.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
[mkl: make first the first patch]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>