ppc4xx: Add AMCC Arches board support (dual 460GT)

The Arches Evaluation board is based on the AMCC 460GT SoC chip.
This board is a dual processor board with each processor providing
independent resources for Rapid IO, Gigabit Ethernet, and serial
communications. Each 460GT has it's own 512MB DDR2 memory, 32MB NOR
FLASH, UART, EEPROM and temperature sensor, along with a shared debug
port. The two 460GT's will communicate with each other via shared
memory, Gigabit Ethernet and x1 PCI-Express.

Signed-off-by: Adam Graham <agraham@amcc.com>
Signed-off-by: Victor Gallardo <vgallardo@amcc.com>
Signed-off-by: Stefan Roese <sr@denx.de>