clk: composite: Export clk_hw_register_composite()

Export clk_hw_register_composite() to support user built as module.

ERROR: modpost: "clk_hw_register_composite" [drivers/clk/imx/mxc-clk.ko]
undefined!

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>