usb: musb: add softconnect for host mode

Add a debugfs interface - softconnect - for host mode to
connect/disconnect the devices without physically remove the
them.

This adds the capability to re-enumerate the devices which are
permanently mounted on the board with the MUSB controller
together.

Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>