[POWERPC] 4xx: Add 405EX CPU type needed for EMAC support on Kilauea

For EMAC support, 405EX needs to be defined to enable the corresponding
EMAC features (IBM_NEW_EMAC_EMAC4, etc.).

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