I've spent some time trying to get the ONF (Watchdog enable status)
bit in the IWDG_SR register to read as something other than zero.

It has since been confirmed to be non-functional[1]. To avoid someone
else spending time on this, document that running status is unsupported
on this hardware explicitly.
No functional change as UNSUPPORTED is already the default.

[1]: https://www.spinics.net/lists/arm-kernel/msg770527.html

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>