All imx usb controller's non core registers uses the same clock gate with
core registers, the usbmisc_imx is the library for imx glue driver, the
glue keeps clock on when it calls usbmisc_imx API to change non-core register.

Besides, we will support runtime pm in the future, it also needs to
close this clock when the usb is not in use.

Philipp Zabel also verifies it at imx6q platform, see
http://www.spinics.net/lists/linux-usb/msg118491.html

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>