clk: move Kconfig options into sub-menu

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/clk/Kconfig b/drivers/clk/Kconfig
index 890f22f..9fcde39 100644
--- a/drivers/clk/Kconfig
+++ b/drivers/clk/Kconfig
@@ -1,3 +1,5 @@
+menu "Clock"
+
 config CLK
 	bool "Enable clock driver support"
 	depends on DM
@@ -17,3 +19,5 @@
 	  SPL, enable this option. It might provide a cleaner interface to
 	  setting up clocks within SPL, and allows the same drivers to be
 	  used as U-Boot proper.
+
+endmenu