usb: dwc3: rockchip: add force dr_mode function

usb: dwc3: rockchip: add force dr_mode function

In current code, the mode of usb is control by extcon or
define in the dts. So in the case of none extcon, we can't
change the usb mode. This patch add a node in debug file
system for the application to change usb mode even if the
extcon is not exist. (P.S. The extcon property must not be
set and dr_mode must be otg if you want to use
rk_usb_force_mode.)

usage:
config host dr_mode:
echo host > sys/kernel/debug/usb@fe800000/rk_usb_force_mode
config peripheral dr_mode:
echo peripheral > sys/kernel/debug/usb@fe800000/rk_usb_force_mode
config otg dr_mode:
echo otg > sys/kernel/debug/usb@fe800000/rk_usb_force_mode

Change-Id: I9cb3b14b7a365fe74e4aa253f0e3f680f45e0f9f
Signed-off-by: Meng Dongyang <daniel.meng@rock-chips.com>