ARM: mxs: use auxdata to pass flexcan_switch function hook

There are some efforts going on to have flexcan_switch function
implemented in flexcan driver, so that this platform function hook
can be saved for device tree boot.  But due to the mx28evk board
design oddness that two flexcan transceivers share one switch gpio,
we have to come up a separate and generic gpio-switch driver to handle
all these gpio switch use cases.

Before that happens, we choose to use auxdata to pass flexcan_switch
function hook, so that the DT conversion is not blocked there.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>