powerpc/kexec: kexec_sequence() is in misc_64.S

Correct reference to the location of the kexec_sequence() assembly helper.
There never was a kexec_stub.S in mainline.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>