[POWERPC] ppc: Add support for AMCC Taishan 440GX eval board

This patch adds support for the AMCC Taishan PPC440GX evaluation
board.

This is still an arch/ppc port. I'm aware that the move of
4xx to arch/powerpc is making good progress right now. So this
patch is mainly intended to make the Taishan support available
for the community right now.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>