clk: Add clk_unregister_{divider, gate, mux} to close memory leak

The common clk_register_{divider,gate,mux} functions allocated memory
for internal data which wasn't freed anywhere. Drivers using these
helpers could only unregister clocks but the memory would still leak.

Add corresponding unregister functions which will release all resources.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@linaro.org>