ARM: imx: enable flexcan on imx25, imx35, imx53, imx6q

Aas 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>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>