ARM: mach-shmobile: Fix headsmp.S code to use CPUINIT

Convert the low level SMP assembly code for SH-Mobile ARM
from using the INIT to the CPUINIT section. This unbreaks
onlining of CPUs using the CPU hotplug interface:

echo 1 > /sys/devices/system/cpu/cpu1/online

Without this fix the reset vector code used by CPU hotplug
will be freed as init section data and CPU cores cannot
be brought online.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>