rtc: imxdi: add some background info about the states the machine can be in

Document the i.MX DryIce machine states.

Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
Signed-off-by: Robert Schwebel <rsc@pengutronix.de>
[rsc: got NDA clearance from Freescale]
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>