ENGR00291876 usb: phy-mxs: add delay before set phyctrl.clkgate

There is a request from IC engineer that if we doesn't
set phypwd as 0xffffffff, we need to delay about five
32Khz cycles before set phypwd, otherwise, the wakeup
signal may can't wake up controller.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
(cherry picked from commit a7a99b979db9d0bf7277533a3a39ba09755768f0)