rtc: imxdi: when locked, do not fail silently

If the DryICE unit is locked it is impossible to set the time. Provide an
error message for this case.

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>