[POWERPC] 4xx: Fix problem with new TLB storage attibute fields on 440x6 core

The new 440x6 core used on AMCC 460EX/GT introduces new storage attibure
fields to the TLB2 word. Those are:

Bit  11   12   13   14   15
     WL1  IL1I IL1D IL2I IL2D

With these bits the cache (L1 and L2) can be configured in a more flexible
way, instruction- and data-cache independently now. The "old" I and W bits
are still available and setting these old bits will automically set these
new bits too (for backward compatibilty).

The current code does not clear these fields resulting in disabling the cache
by chance. This patch now makes sure that these new bits are cleared when
the TLB2 word is written.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>