ARM: imx: enable big endian mode

Enable ARM big-endian mode on mach-imx. This requires adding some
byte swapping in the debug functions (which otherwise hang forever)
and of course the secondary core bringup.

Tested (on top of 4.4) on i.MX6 HummingBoard quad-core (IMX6Q).

The patch is pretty much as suggested by Arnd Bergmann, thanks!

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>