rtc: add rtc command

Mostly as an aid for debugging RTC drivers, provide a command that can
be used to read/write arbitrary registers (assuming the driver
provides the read/write methods or their single-register-at-a-time
variants).

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 2b823dd..846c905 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1739,6 +1739,12 @@
 	  Enable the 'date' command for getting/setting the time/date in RTC
 	  devices.
 
+config CMD_RTC
+	bool "rtc"
+	depends on DM_RTC
+	help
+	  Enable the 'rtc' command for low-level access to RTC devices.
+
 config CMD_TIME
 	bool "time"
 	help