thermal: Added Bang-bang thermal governor

The bang-bang thermal governor uses a hysteresis to switch abruptly on
or off a cooling device.  It is intended to control fans, which can
not be throttled but just switched on or off.
Bang-bang cannot be set as default governor as it is intended for
special devices only.  For those special devices the driver needs to
explicitely request it.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Andreas Mohr <andi@lisas.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Javi Merino <javi.merino@arm.com>
Cc: linux-pm@vger.kernel.org
Signed-off-by: Peter Feuerer <peter@piie.net>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
diff --git a/drivers/thermal/Kconfig b/drivers/thermal/Kconfig
index 693208e..2500ecc 100644
--- a/drivers/thermal/Kconfig
+++ b/drivers/thermal/Kconfig
@@ -84,6 +84,16 @@
 	  Enable this to manage platform thermals using a simple linear
 	  governor.
 
+config THERMAL_GOV_BANG_BANG
+	bool "Bang Bang thermal governor"
+	default n
+	help
+	  Enable this to manage platform thermals using bang bang governor.
+
+	  Say 'Y' here if you want to use two point temperature regulation
+	  used for fans without throttling.  Some fan drivers depend on this
+	  governor to be enabled (e.g. acerhdf).
+
 config THERMAL_GOV_USER_SPACE
 	bool "User_space thermal governor"
 	help