watchdog: Convert to using sbintime_t format

[jhibbits]

Summary

Some watchdogs are now based on a countdown timer instead of a bit
check. To deal with these, convert the watchdog framework to use
sbintime_t instead of power-of-2-nanoseconds. This allows more
precision, and more variety of watchdog timeouts. Keep the old method
as a compatibility layer, so that drivers can be migrated slowly, as
needed.

[jhb]

sys/dev/watchdog/watchdog.c

198

Spurious line?

390

Hmm, you could just let these all fall through to the default case for now. Even if you want explicit case statements maybe just use:

case WDIOC_GETTIMEOUT:
case WDIOC_GETTIMELEFT:
...
default:
           error = ENIOCTL;

[jhibbits]

Address feedback.

[jhb]

This looks ok to me.