arm: use processor specififc functions to turn off MMU
The way it was done before does not work on Cortex processors.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Orjan Friberg <of@flatfrog.com>
Tested-by: Luca Ceresoli <list@lucaceresoli.net>