powerpc/4xx: MTD support for the AMCC Taishan Board

Add support for the NOR flash found on the AMCC Taishan Board
and enable MTD support in the defconfig.

Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>