Cortex-A: Don't flush the data/unified cache if MMU is off

Cortex-A: Don't flush the data/unified cache if MMU is off

When the SCTLR has C set but M unset (i.e. Caching on, but MMU off) the cache
if effectively off. So only flush the cache if MMU is on, otherwise stale
entries might be committed to memory.

Change-Id: Iaff8b6f25b7a41ba838b91d45684c98f99fc0b27
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-on: http://openocd.zylin.com/2429
Tested-by: jenkins
Reviewed-by: Christopher Head <chead@zaber.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Vladimir Svoboda <ze.vlad@gmail.com>