ARM i.MX: introduce imx_otg_ulpi_create to create ULPI transceivers

The boards are currently using otg_ulpi_create and mxc_ulpi_access_ops,
both are only present if CONFIG_USB_ULPI is set. To remove the need of
ifdefs in the board code introduce a imx_otg_ulpi_create functions
which expands to a static inline function if compiled without ulpi.

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