powerpc/85xx: cuboot - Fix up ethernet3 MAC address on MPC85xx

Some MPC85xx platforms do support 4 ethernet ports, so make sure the boot
wrapper fixes up all of them in the fdt.

Since MAC addresses are at the end of the bd_t structure there is no harm
in expanding to support 4 MAC address on older 85xx systems that might not
have that many.

Signed-off-by: Nate Case <ncase@xes-inc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>