powerpc/kexec: Reset secondary cpu endianness before kexec

If the target kernel does not inlcude the FIXUP_ENDIAN check, coming
from a different-endian kernel will cause the target kernel to panic.
All ppc64 kernels can handle starting in big-endian mode, so return to
big-endian before branching into the target kernel.

This mainly affects pseries as secondaries on powernv are returned to
OPAL.

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>