BACKPORT: 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!

(cherry picked from commit d7d7518fdcc8f43da69fb40073273e77f24be24f)
[Jindong: Resolved minor conflict in drivers/clk/clk-composite.c ]
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Jindong Yue <jindong.yue@nxp.com>
Change-Id: I54496d44d4bf6599af3a3ec9c40c02556d55cf17