clk: Add clk_hw_unregister_composite helper function definition

This function has been delcared but not defined anywhere. Hence, this
commit adds definition for it.

Fixes: 49cb392d

 ("clk: composite: Add hw based registration APIs")
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lkml.kernel.org/r/20191115162901.17456-3-manivannan.sadhasivam@linaro.org


Signed-off-by: Stephen Boyd <sboyd@kernel.org>