timers/mcp794xx: add possibility to store datetime in UTC

This commit adds configuration option CONFIG_MCP794XX_DATETIME_UTC.
If set, the datetime is stored in UTC instead of local time. The
default value is kept at local time to keep backwards compatibility
with devices currently using the RTC.

Signed-off-by: Michal Lenc <michallenc@seznam.cz>

drivers/timers/Kconfig

@@ -339,6 +339,13 @@ config RTC_MCP794XX
 
 if RTC_MCP794XX
 
+config MCP794XX_DATETIME_UTC
+	bool "Store datetime in UTC"
+	default n
+	---help---
+		If set, the datetime is stored in UTC timezone instead of timezone
+		defined by local time.
+
 config MCP794XX_I2C_FREQUENCY
 	int "MCP794XX I2C frequency"
 	default 400000

drivers/timers/mcp794xx.c

@@ -407,11 +407,23 @@ int up_rtc_settime(FAR const struct timespec *tp)
       newtime++;
     }
 
+#ifndef CONFIG_MCP794XX_DATETIME_UTC
+  /* Save datetime in local time. */
+
   if (localtime_r(&newtime, &newtm) == NULL)
     {
       rtcerr("ERROR: localtime_r failed\n");
       return -EINVAL;
     }
+#else
+  /* Save datetime in UTC time. */
+
+  if (gmtime_r(&newtime, &newtm) == NULL)
+    {
+      rtcerr("ERROR: gmtime_r failed\n");
+      return -EINVAL;
+    }
+#endif
 
   rtc_dumptime(&newtm, "New time");