riowatchdog: Convert to pure OF driver.

This also cleans up a lot of crud in this driver:

1) Don't touch the BBC regs, just leave the watchdog trigger
   behavior whatever the firmware programmed it to.

2) Use WATCHDOG_MINOR instead of hardcoded and not properly
   allocated RIOWD_MINOR.

Hey, I haven't touched it since I wrote it years ago :-)

Signed-off-by: David S. Miller <davem@davemloft.net>