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: Shawn Guo <shawn.guo@linaro.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>