usb: otg-fsm: Add documentation for struct otg_fsm

struct otg_fsm is the interface to the OTG state machine.

Document the input, output and internal state variables.
Definations are taken from Table 7-2 and Table 7-4 of
the USB OTG & EH Specification Rev.2.0

Re-arrange some of the members as per use case for more
clarity.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>