clk: add config option HAVE_CLK_PREPARE into Kconfig

The commit 40d3e0f4 (clk: provide prepare/unprepare functions) provides
the config option HAVE_CLK_PREPARE for platforms that have
prepare/unprepare implementation to select.  This patch adds the option
into drivers/clk/Kconfig, so that it can be actually selected.
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>

drivers/clk/Kconfig

@@ -3,5 +3,8 @@ config CLKDEV_LOOKUP
 	bool
 	select HAVE_CLK
 
+config HAVE_CLK_PREPARE
+	bool
+
 config HAVE_MACH_CLKDEV
 	bool