dt-bindings: tcan4x5x: Make wake-gpio an optional gpio

The wake-up of the device can be configured as an optional feature of
the device. Move the wake-up gpio from a requried property to an
optional property.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Cc: Rob Herring <robh@kernel.org>
Reviewed-by: Sean Nyekjaer <sean@geanix.com>
Tested-by: Sean Nyekjaer <sean@geanix.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>