MIPS: Loongson-3: fix compile mips cpu_hwmon as module build error.

  set cpu_hwmon as a module build with loongson_sysconf, loongson_chiptemp
  undefined error,fix cpu_hwmon compile options to be bool.Some kernel
  compilation error information is as follows:

  Checking missing-syscalls for N32
  CALL    scripts/checksyscalls.sh
  Checking missing-syscalls for O32
  CALL    scripts/checksyscalls.sh
  CALL    scripts/checksyscalls.sh
  CHK     include/generated/compile.h
  CC [M]  drivers/platform/mips/cpu_hwmon.o
  Building modules, stage 2.
  MODPOST 200 modules
ERROR: "loongson_sysconf" [drivers/platform/mips/cpu_hwmon.ko] undefined!
ERROR: "loongson_chiptemp" [drivers/platform/mips/cpu_hwmon.ko] undefined!
make[1]: *** [scripts/Makefile.modpost:92：__modpost] 错误 1
make: *** [Makefile:1261：modules] 错误 2
Signed-off-by: Yupeng Li <liyupeng@zbhlos.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

drivers/platform/mips/Kconfig

@@ -17,7 +17,7 @@ menuconfig MIPS_PLATFORM_DEVICES
 if MIPS_PLATFORM_DEVICES
 
 config CPU_HWMON
-	tristate "Loongson-3 CPU HWMon Driver"
+	bool "Loongson-3 CPU HWMon Driver"
 	depends on MACH_LOONGSON64
 	select HWMON
 	default y