ppc4xx: Add device tree support to AMCC Yosemite

Add support for booting with a device tree blob. This is needed to boot
ARCH=powerpc kernels. Also add support for setting the eth0 mac address
via the ethaddr variable.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Stefan Roese <sr@denx.de>