[POWERPC] 4xx: Base support for 440GX Taishan eval board

This patch adds base support for the AMCC Taishan 440GX evaluation
board.

Signed-off-by: Hugh Blemings <hugh@blemings.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>