clk: tegra: Implement locking for super clock

Although tegra_clk_register_super_mux() has a lock parameter, the lock is not
actually used by the code. Fixed with this patch.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Reviewed-by: Prashant Gaikwad <pgaikwad@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>