arm946e: cp15 command returns value to the script

arm946e: cp15 command returns value to the script

Not just print it.

This enables scripts to analyze valuable config options of
arm946e-s cores, do internal BIST memory tests and more.

Be careful to flush caches before disabling it.
Do not forget that BIST test overwrites  memory.

 - cp15 rewritten from COMMAND_HANDLER to jim_handler.

Change-Id: I734da0be6db0a3127c2daa94ed75efef94da8ceb
Signed-off-by: Alexander Osipenko <sipych@gmail.com>
Reviewed-on: http://openocd.zylin.com/694
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>