rtc: Allow use of RTC in SPL and TPL

Add Kconfig options so that the RTC can be used in SPL and TPL. This is
helpful for accessing the contents of CMOS RAM, for example.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/rtc/Makefile b/drivers/rtc/Makefile
index 513e3ff..1724602 100644
--- a/drivers/rtc/Makefile
+++ b/drivers/rtc/Makefile
@@ -4,7 +4,7 @@
 # Wolfgang Denk, DENX Software Engineering, wd@denx.de.
 #ccflags-y += -DDEBUG
 
-obj-$(CONFIG_DM_RTC) += rtc-uclass.o
+obj-$(CONFIG_$(SPL_TPL_)DM_RTC) += rtc-uclass.o
 
 obj-$(CONFIG_RTC_AT91SAM9_RTT) += at91sam9_rtt.o
 obj-y += date.o