This adds a barrier after setup_c(). This is necessary because otherwise
some global variable assignments may be reordered by the compiler to be
executed before setup_c which cannot work.
This was observed when doing other unrelated changes to the start function,
it seems in current mainline state the compiler does not actually reorder
the code.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>