usb: gadget: udc: renesas_usb3: add support for usb role swap

This patch adds support for usb role swap via sysfs "role".

For example:
 1) Connect a usb cable using 2 Salvator-X boards.
  - For A-Device, the cable is connected to CN11 (USB3.0 ch0).
  - For B-Device, the cable is connected to CN9 (USB2.0 ch0).
 2) On A-Device, you input the following command:
  # echo peripheral > /sys/devices/platform/soc/ee020000.usb/role
 3) On B-Device, you input the following command:
  # echo host > /sys/devices/platform/soc/ee080200.usb-phy/role

Then, the A-Device acts as a peripheral and the B-Device acts as
a host. Please note that A-Device must input the following command
if you want the board to act as a host again.
 # echo host > /sys/devices/platform/soc/ee020000.usb/role

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Documentation/ABI/testing/sysfs-platform-renesas_usb3

@@ -0,0 +1,15 @@
+What:		/sys/devices/platform/<renesas_usb3's name>/role
+Date:		March 2017
+KernelVersion:	4.13
+Contact:	Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
+Description:
+		This file can be read and write.
+		The file can show/change the drd mode of usb.
+
+		Write the following string to change the mode:
+		 "host" - switching mode from peripheral to host.
+		 "peripheral" - switching mode from host to peripheral.
+
+		Read the file, then it shows the following strings:
+		 "host" - The mode is host now.
+		 "peripheral" - The mode is peripheral now.