imx: dynamically register flexcan devices for mx25 and mx35

In order to make this patch compile, even if the flexcan driver with
it's header defining the platform data isn't merged yet, two ifdefs have
been added. They effect that the "imx_add_flexcan" function results in a
no-op function if the driver hasn't been activated.

These ifdefs can be removed after the flexcan driver has been merged.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>