dm: rtc: Add a uclass for real-time clocks

Add a uclass for real-time clocks which support getting the current time,
setting it and resetting the chip to a known-working state. Some RTCs have
additional registers which can be used to store settings, so also provide
an interface to these.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/uclass-id.h b/include/dm/uclass-id.h
index 395e25a..08f1bad 100644
--- a/include/dm/uclass-id.h
+++ b/include/dm/uclass-id.h
@@ -46,6 +46,7 @@
 	UCLASS_USB_DEV_GENERIC,	/* USB generic device */
 	UCLASS_MASS_STORAGE,	/* Mass storage device */
 	UCLASS_CPU,		/* CPU, typically part of an SoC */
+	UCLASS_RTC,		/* Real time clock device */
 
 	UCLASS_COUNT,
 	UCLASS_INVALID = -1,