Unlike barebox, U-Boot's go command doesn't have any provisions
for cache handling and as such, bootstrapping barebox via go can
cause stale data to be erroneously executed as instructions.

The official documentation[1] suggests use of bootm instead, which
does the necessary flushing and invalidation. Update our documentation
accordingly.

[1]: http://www.denx.de/wiki/view/DULG/UBootStandalone#Section_5.12.3.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>