MIPS: Loongson-3: Add chipset ACPI platform driver

This add south-bridge (SB700/SB710/SB800 chipset) ACPI platform driver
for Loongson-3. This will be used by EC (Embedded Controller, used by
laptops) driver and STR (Suspend To RAM).

[ralf@linux-mips.org: Fix build error if !CONFIG_CPU_LOONGSON3.  Build
doesn't like it if no obj-* variable is defined at all in a Makefile.
Obviously this has not been tested on other platforms.]

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/9619/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>