usb: dwc3: add functions to set force mode

usb: dwc3: add functions to set force mode

Add functions to set force mode for host and device.
These functions will check the current mode and only
force if needed thus avoiding unnecessary force mode
delays. It's useful for dwc3 controller on some platforms
which don't support otg mode but support drd mode,
like rk3399 platform.

TEST=config dr_mode = "otg" in dts first, and do
"echo device > mode" on usb debugfs dir to force device mode,
do "echo host > mode" on usb debugfs dir to force host mode.

Change-Id: I1f90ac9d1ee3daa19c1046b0e52fdfb8f2d2ad62
Signed-off-by: Wu Liang feng <wulf@rock-chips.com>