rtc:ds3232/ds3231: Add support to generate 32KHz output

RTC devices can generate 32KHz output if for
-DS3232 device, EN32KHz bit and BB32KHz bit are set
-DS3231 device, EN32KHz bit is set, BB32KHz bit is don't care

Patch adds rtc_enable_32khz_output() which when called
will enable 32KHz output on 32KHz pin

Signed-off-by: Priyanka Jain <Priyanka.Jain@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>
diff --git a/include/rtc.h b/include/rtc.h
index bd8621d..69fe8d4 100644
--- a/include/rtc.h
+++ b/include/rtc.h
@@ -151,6 +151,7 @@
 int rtc_get (struct rtc_time *);
 int rtc_set (struct rtc_time *);
 void rtc_reset (void);
+void rtc_enable_32khz_output(void);
 
 /**
  * rtc_read8() - Read an 8-bit register