phy: rcar-gen3-usb2: add sysfs for usb role swap

This patch adds sysfs "role" for usb role swap. This parameter can be
read and write. If you use this file as the following, you can swap
the usb role.

For example:
 1) Connect a usb cable using 2 Salvator-x boards
 2) On A-Device (ID pin is low), you input the following command:
   # echo peripheral > /sys/devices/platform/soc/ee080200.usb-phy/role
 3) On B-Device (ID pin is high), 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. (even if you disconnect the usb
cable, since id state may be the same, the A-Device keeps to act as
peripheral.)
 # echo host > /sys/devices/platform/soc/ee080200.usb-phy/role

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>