ARM i.MX6: change mxs usbphy clock usage

This mxs usbphy is only needs to be on after system boots
up, and software never needs to control it anymore.
Meanwhile, usbphy's parent needs to be notified if usb
is suspend or not. So we design below mxs usbphy usage:

- usbphy1_gate and usbphy2_gate:
Their parents are dummy clock, we only needs to enable
it after system boots up.
- usbphy1 and usbphy2
Usage reserved bit for this clock, in that case, the refcount
will be updated, but without hardware changing.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>