rtc: ds1307: add temperature sensor support for ds3231

DS3231 has the temperature registers with a resolution of 0.25
degree celsius.  This enables to get the value through hwmon.

	# cat /sys/class/i2c-adapter/i2c-2/2-0068/hwmon/hwmon0/temp1_input
	21000

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index 27f6ac4..d47ad96 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -212,6 +212,15 @@
 	  This driver can also be built as a module. If so, the module
 	  will be called rtc-ds1307.
 
+config RTC_DRV_DS1307_HWMON
+	bool "HWMON support for rtc-ds1307"
+	depends on RTC_DRV_DS1307 && HWMON
+	depends on !(RTC_DRV_DS1307=y && HWMON=m)
+	default y
+	help
+	  Say Y here if you want to expose temperature sensor data on
+	  rtc-ds1307 (only DS3231)
+
 config RTC_DRV_DS1374
 	tristate "Dallas/Maxim DS1374"
 	help