powerpc/boot: Add a global entry point for pseries

When entering the boot wrapper in little endian, we will need to fix
the endian order using a fixup trampoline like in the kernel. This
patch overrides the _zimage_start entry point for this purpose.

Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

diff --cc arch/powerpc/boot/Makefile
Simple merge

diff --cc arch/powerpc/boot/pseries-head.S
Simple merge

diff --cc arch/powerpc/boot/wrapper
Simple merge