rtc: Add SPI alias for pcf2123 driver

Without this alias, Device Tree won't cause the driver
to be loaded.

See: #1510

drivers/rtc/rtc-pcf2123.c

@@ -358,3 +358,4 @@ MODULE_AUTHOR("Chris Verges <chrisv@cyberswitching.com>");
 MODULE_DESCRIPTION("NXP PCF2123 RTC driver");
 MODULE_LICENSE("GPL");
 MODULE_VERSION(DRV_VERSION);
+MODULE_ALIAS("spi:rtc-pcf2123");