ARM i.MX ehci: factor out soc specific functions

Currently we have a mxc_initialize_usb_hw which is called on every
i.MX SoC. This function dispatches the different SoC types, which
is quite ugly. This patch moves the SoC specific USB initialization
to their correspondive mach directories.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>