usb: musb: add ulpi access operations

This adds helper functions for ULPI access, and implements
otg_io_access_ops for musb.

Signed-off-by: Heikki Krogerus <ext-heikki.krogerus@nokia.com>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>