powerpc/8xx: CONFIG_PIN_TLB unneeded for CONFIG_PPC_EARLY_DEBUG_CPM

IMMR is now mapped by a fixed 512k page managed by the TLB miss
handler so it is not anymore necessary to PIN TLBs

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>