powerpc: mpc8xx: cleaning up watchdog

In preparation of migration to DM watchdog, clean up a bit.

The 8xx watchdog really is a HW watchdog, so declare it as is
then it goes through Kconfig

And the watchdog reset doesn't mind getting interrupted, so
no need to disable interrupts

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>