ppc4xx: Generic architecture for xilinx ppc405(v3)

As "ppc44x: Unification of virtex5 pp440 boards" did for the xilinx
ppc440 boards, this patch presents a common architecture for all the
xilinx ppc405 boards.

Any custom xilinx ppc405 board can be added very easily with no code
duplicity.

This patch also adds a simple generic board, that can be used on almost
any design with xilinx ppc405 replacing the file ppc405-generic/xparameters.h

This patch is prepared to work with the latest version of EDK (10.1)

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@uam.es>
Signed-off-by: Stefan Roese <sr@denx.de>