rtc: ds3232: add temperature support

DS3232/DS3234 has the temperature registers with a resolution of 0.25
degree celsius. This enables to get the value through hwmon.

	# cat /sys/class/hwmon/hwmon0/temp1_input
	37250

Signed-off-by: Kirill Esipov <yesipov@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>