powerpc: mpc8xx: get rid of the multiple PVR_ values

Avoid hardcoding the PVR values in C since they are defined
in processor.h

At the same time, remove those multiple PVR values for 8xx and
keep only one that we call PVR_8xx

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