MIPS: OCTEON: fix kexec support

Commit 62cac480 ("MIPS: kexec: Make a framework for both jumping and
halting on nonboot CPUs") broke the build of the OCTEON platform as
the relocated_kexec_smp_wait() is now static and not longer exported in
kexec.h.

Replace it by kexec_reboot() like it has been done in other places.

Fixes: 62cac480 ("MIPS: kexec: Make a framework for both jumping and halting on nonboot CPUs")
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Dengcheng Zhu <dzhu@wavecomp.com>
Cc: ralf@linux-mips.org
Cc: stable@vger.kernel.org # 4.20+