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/drivers/rtc/Makefile b/drivers/rtc/Makefile
index fdcbc00..61373b6 100644
--- a/drivers/rtc/Makefile
+++ b/drivers/rtc/Makefile
@@ -7,6 +7,8 @@
 
 #ccflags-y += -DDEBUG
 
+obj-$(CONFIG_DM_RTC) += rtc-uclass.o
+
 obj-$(CONFIG_RTC_AT91SAM9_RTT) += at91sam9_rtt.o
 obj-$(CONFIG_RTC_BFIN) += bfin_rtc.o
 obj-y += date.o