ARM: mx25: fix offset for usb host controller

In commit 2c20b9f (ARM: mx25: dynamically allocate mxc-ehci devices) I
changed the offset to the value specified in the reference manual
intending to test this change on hardware.  This slipped through and now
prooved to be wrong.  So fix it and add a comment about the
documentation being wrong.

Reported-by: Jaume Ribot <jaume@fqingenieria.es>
Cc: Michael Trimarchi <trimarchi@gandalf.sssup.it>
Cc: Shawn Guo <shawn.gsc@gmail.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>