ARM: mx35: Add mx35_revision function to query the silicon revision

Based on work done earlier by Sascha Hauer

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Eric Bénard <eric@eukrea.com>
[ukl: actually squash the two approaches together]
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>