powerpc/smp: add generic_set_cpu_up() to set cpu_state as CPU_UP_PREPARE

In the case of cpu hotplug, the cpu_state should be set to CPU_UP_PREPARE
when kicking cpu.  Otherwise, the cpu_state is always CPU_DEAD after
calling generic_set_cpu_dead(), which makes the delay in generic_cpu_die()
not happen.

Signed-off-by: Zhao Chenhui <chenhui.zhao@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>