rtc: move date.c from drivers/rtc/ to lib/

In the next commit, rtc_mktime(), for compatibility with linux, will be
implemented using rtc_mktime(), which is no longer drivers/rtc specific.
So move this file under lib/.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index a2b9e19..bc8318d 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1625,6 +1625,7 @@
 config CMD_DATE
 	bool "date"
 	default y if DM_RTC
+	select LIB_DATE
 	help
 	  Enable the 'date' command for getting/setting the time/date in RTC
 	  devices.