arm926ejs: fix write memory operations with caches enabled

Perform proper ICache flush operations on memory writes. This should fix
inability to use software breakpoints for debugging with caches
enabled.

This patch is only compile-time tested.

Commit 1137eaedaf6498f3448cdedf6f93076d9b3fd58a fixed the same issue
for arm920t. Among all the arm7_9_common targets only arm926ejs seems
to be broken in the same way.

Change-Id: I575306ac4319a69fc637b42f7c958f4595c5e81f
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1912
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>