powerpc/476: define specific cpu table entry DD2 core

The DD2 core still has some unstability.  Define CPU_FTR_476_DD2 to
enable workarounds in later patches.

This is based on an earlier, unreleased patch for DD1 by Ben Herrenschmidt.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>