[ar71xx] fix watchdog to work on subsequent SoCs

The AR9341 AHB runs at 225MHz, much faster than the 33MHz of the
AR71xx AHB. So not only is the math going to do weird things, it
will also wrap rather than being clamped.

So:

• clamp! don't wrap!
• tidy up some debugging
• add an option to throw an NMI rather than reset!

Tested:

• AR9341 SoC (TP-Link TL-WDR4300), patting/not patting the watchdog!