watchdog: mpc8xx: Rename it mpc8xxx

mpc8xx, mpc83xx and mpc86xx have similar watchdog with almost same
memory registers.

Rename it mpc8xxx which is the generic name used for drivers supporting
several mpc families.

The driver will be made more generic in following patch.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>