ARM: mxs: enable flexcan on imx28

As the flexcan driver has DT binding for some time now. This patch makes
the flexcan driver available in the can drivers menu if a SOC with flexcan
is enabled.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>