powerpc/zImage: make the "OF" wrapper support ePAPR boot

This makes the "OF" zImage wrapper (zImage.pseries, zImage.pmac,
zImage.maple) work if booted via a flat device-tree (ePAPR boot
mode), and thus potentially usable with kexec.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>