ARM: mx25: dynamically allocate mxc-ehci devices

According to the reference manual of the i.MX25 the host controller uses an
offset of 0x200 not 0x400 as was specified in the resources for mxc_usbh2.

Needs-Testing: yes
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>