phy: rockchip-inno-usb3: support u3 to work on u2 only mode

phy: rockchip-inno-usb3: support u3 to work on u2 only mode

Current code set usb2.0 only mode by type in command and may be
failed if change to usb3.0 for that we need reset the controller
when change mode. This patch add a node in debug file system for
config usb2.0 only or usb2.0/usb3.0 mode. So SLT testing or anyone
else can use this node to change config mode.

1. Config to usb2.0/usb3.0 mode:
   echo u3 > /sys/kernel/debug/<phy name>/u3phy_mode

2. Config to usb2.0 only mode:
   echo u2 > /sys/kernel/debug/<phy name>/u3phy_mode

Change-Id: I11338d8307e771b7d76b61a91477d353444c011c
Signed-off-by: William Wu <wulf@rock-chips.com>