Add CFG_RTC_DS1337_NOOSC to turn off OSC output

The default settings for RTC DS1337 keeps the OSC
output, 32,768 Hz, on. This add CFG_RTC_DS1337_NOOSC to
turn it off.
Signed-off-by: NJoakim Tjernlund <Joakim.Tjernlund@transmode.se>

README

@@ -689,6 +689,7 @@ The following options need to be configured:
 		CONFIG_RTC_DS164x	- use Dallas DS164x RTC
 		CONFIG_RTC_ISL1208	- use Intersil ISL1208 RTC
 		CONFIG_RTC_MAX6900	- use Maxim, Inc. MAX6900 RTC
+		CFG_RTC_DS1337_NOOSC	- Turn off the OSC output for DS1337
 
 		Note that if the RTC uses I2C, then the I2C interface
 		must also be configured. See I2C Support, below.

drivers/rtc/ds1337.c

@@ -158,11 +158,18 @@ void rtc_set (struct rtc_time *tmp)
  * SQW/INTB* pin and program it for 32,768 Hz output. Note that
  * according to the datasheet, turning on the square wave output
  * increases the current drain on the backup battery from about
- * 600 nA to 2uA.
+ * 600 nA to 2uA. Define CFG_RTC_DS1337_NOOSC if you wish to turn
+ * off the OSC output.
  */
+#ifdef CFG_RTC_DS1337_NOOSC
+ #define RTC_DS1337_RESET_VAL \
+          (RTC_CTL_BIT_INTCN | RTC_CTL_BIT_RS1 | RTC_CTL_BIT_RS2)
+#else
+ #define RTC_DS1337_RESET_VAL (RTC_CTL_BIT_RS1 | RTC_CTL_BIT_RS2)
+#endif
 void rtc_reset (void)
 {
-	rtc_write (RTC_CTL_REG_ADDR, RTC_CTL_BIT_RS1 | RTC_CTL_BIT_RS2);
+	rtc_write (RTC_CTL_REG_ADDR, RTC_DS1337_RESET_VAL);
 }