ppc4xx: Add PPC405EX(r) Rev D support

Unfortunately some Rev D PPC405EX/405EXr PVR's are identical with older
405EX(r) parts. Here a list:

0x12911475 - 405EX Rev D with Security *and* 405EX Rev A/B witout Sec
0x12911473 - 405EX Rev D without Security *and* 405EXr Rev A/B with Sec

Since there are only a few older parts in the field, this patch now
changes the PVR's above to represent the new Rev D versions.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Phong Vo" <pvo@amcc.com>