ppc4xx: Add NAND booting support for AMCC Bamboo (440EP) eval board

This patch adds NAND booting support for the AMCC Bamboo eval board.
Since the NAND-SPL boot image is limited to 4kbytes, this version
only supports the onboard 64MBytes of DDR. The DIMM modules can't be
supported, since the setup code for I2C DIMM autodetection and
configuration is too big for this NAND bootloader.

Signed-off-by: Stefan Roese <sr@denx.de>