mx5: fix get_reset_cause

commit d9f43c8f sets
get_reset_cause() as static, but this conflicts with mx5
where its prototype is in sys_proto.h.

Drop it from sys_proto.h and drop print_cpuinfo from mx53_loco,
factorizing the call for this board.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Jason Liu <jason.hui@linaro.org>