powerpc/boot/wrapper: use the pseries wrapper for zImage.epapr

We'll likely be entering the zImage.epapr as BE, so include the pseries
implementation of _zimage_start, which adds the endian fixup magic.

Although the endian fixup won't work on Book III-E machines starting LE,
the current entry point doesn't support LE anyway, so we shouldn't be
breaking anything.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>