ARM: imx: change the way flexcan devices are registered

Group soc specific data in a global struct instead of repeating it for each
call to imxXX_add_flexcanX.  The structs holding the actual data are placed
in .init.constdata and so don't do much harm.  Compared to the previous
approach this reduces code size to call imx_add_flexcan.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>