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.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Acked-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Scott Wood <scottwood@freescale.com>