powerpc/8xx: Fixing memory init issue with CONFIG_PIN_TLB
Activating CONFIG_PIN_TLB allows access to the 24 first Mbytes of
memory at bootup instead of 8. It is needed for "big" kernels for
instance when activating CONFIG_LOCKDEP_SUPPORT. This needs to be
taken into account in init_32 too, otherwise memory allocation soon
fails after startup.