MIPS: Loongson-3: Add CSR IPI support

MIPS: Loongson-3: Add CSR IPI support

CSR IPI and legacy MMIO use the same infrastructure, but CSR IPI is
faster than legacy MMIO IPI. This patch enable CSR IPI if possible
(except for MailBox, because CSR IPI is too complicated for MailBox).

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: linux-mips@vger.kernel.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Cc: Huacai Chen <chenhuacai@gmail.com>