powerpc/8xx: Fixing issue with CONFIG_PIN_TLB

Activating CONFIG_PIN_TLB is supposed to pin the IMMR and the first
three 8Mbytes pages.  But the setting of MD_CTR to a pinnable entry was
missing before the pinning of the third 8Mb page.  As the index is
decremented module 28 (MD_RSV4D is set) after every DTLB update, the
third 8Mbytes page was not pinned.

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