ARM: hi3xxx: add hotplug support

Enable hotplug support on hi3xxx platform

How to test:
cat proc/interrupts
echo 0 > /sys/devices/system/cpu/cpuX/online
cat proc/interrupts
echo 1 > /sys/devices/system/cpu/cpuX/online

Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Tested-by: Zhang Mingjun <zhang.mingjun@linaro.org>
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
[khilman: fixed checkpatch error]
Signed-off-by: Kevin Hilman <khilman@linaro.org>