csky: SMP support

This patch adds boot, ipi, hotplug codes for SMP.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>