[POWERPC] 4xx: Enable EMAC for PPC405 Walnut board

This patch enables the ibm_newemac driver for the Walnut board.  It fixes the
device tree for the walnut board to order the MAL interrupts correctly and
adds the local-mac-address property to the EMAC node.  The bootwrapper is also
updated to extract the MAC address from the OpenBIOS offset where it is stored.

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