samples: drivers: rtc: change hard-coded device name

Change to use the device config name defined by driver's Kconfig
for device binding, instead of hard-coding it which is not
portable.

Jira: ZEP-1764

Change-Id: I7af234ada73302eb062340740df2fc7a8539150d
Signed-off-by: Kuo-Lang Tseng <[email protected]>
(cherry picked from commit 1980325)

Author: kltseng

samples/drivers/rtc/src/main.c

@@ -26,7 +26,7 @@ void main(void)
 	struct device *rtc_dev;
 
 	printk("Test RTC driver\n");
-	rtc_dev = device_get_binding("RTC_0");
+	rtc_dev = device_get_binding(CONFIG_RTC_0_NAME);
 
 	config.init_val = 0;
 	config.alarm_enable = 1;