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/lib/Kconfig b/lib/Kconfig
index 55dedcb..2c018cb 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -574,4 +574,7 @@
 	bool "enable fdtdec test"
 	depends on OF_LIBFDT
 
+config LIB_DATE
+	bool
+
 endmenu