mx25: fix the offset between the USB ports' registers

The USBOH module on imx25 chips contains two USB controllers which are
called USB OTG Controller and USB Host Controller. Each one has its EHCI
root hub. The OTG Controller's EHCI registers start at offset 0, the Host
Controller's registers start at offset 0x400.

We set CONFIG_MXC_USB_PORT=0 to select the OTG Controller and 1 for the
Host Controller. Therefore, IMX_USB_PORT_OFFSET must be 0x400. Using
this setting, the Host Controller starts working on my imx25 board.

Please note that the imx25 reference manual claims that the Host
Controller's registers start at 0x200. This is not correct. The Linux
Kernel uses the correct offset 0x400 in imx25.dtsi.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>