clk: Add __clk_hw_set_clk helper function

After the clk API change to return a per-user clock instance, both the
struct clk_core and struct clk pointers from the hw clock needs to be
assigned to clock that share the same state.

In the future the struct clk_core will be removed and this is going to
change again so to avoid having to change the assignments twice in all
the drivers, add a helper function to have an indirection level.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@linaro.org>