powerpc/watchdog: Don't enable interrupt on PPC64 BookE

Critical interrupts are not handled on PPC64 BookE machines,
so when the first watchdog interrupt fires the machine will
freeze without a warning until it's rebooted by the second
watchdog trigger.
Plus, the interrupt isn't used anyway since the driver
expects a usermode app to ping the watchdog periodically.

Signed-off-by: Laurentiu Tudor <Laurentiu.Tudor@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>