usb: gadget: udc: renesas_usb3: add debugfs to set the b-device mode

This patch adds debugfs to set the "b-device" mode for using a board
which is not connected to the ID pin (e.g. CN11 on Salvator-X).
If we want to use peripheral mode on such a board, we have to disable
VBUS output first. So, this patch can set such a mode as the following:

 # mount -t debugfs none /sys/kernel/debug
 # modprobe renesas_usb3
 # modprobe g_mass_storage file=/dev/shm/test.bin
 # echo 1 > /sys/kernel/debug/ee020000.usb/b_device

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