usb: gadget: fsl_mxc_udc: do not depend on grouped clocks

With the new common clock infrastructure, the following clocks should be
used on i.MX drivers: ipg, per and ahb.

Adapt fsl_mxc_udc to follow this new behaviour to fix the following probe error:

Freescale High-Speed USB SOC Device Controller driver (Apr 20, 2007)
fsl-usb2-udc fsl-usb2-udc: clk_get("usb") failed
fsl-usb2-udc: probe of fsl-usb2-udc failed with error -2

Reported-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>