powerpc/boot: fix global entry point for pseries

Patch 15/18 "powerpc/boot: add a global entry point for pseries"
introduced a pseries-head.S file to fix the endian order when
entering the boot wrapper. This patch is a fix for some platforms
which also generate the zImage.pseries boot wrapper.

Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>