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/Makefile b/lib/Makefile
index e15a189..90d4425 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -117,4 +117,6 @@
 obj-y += vsprintf.o strto.o
 endif
 
+obj-y += date.o
+
 subdir-ccflags-$(CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED) += -O2