clk: Add clk_hw_get_clk() helper API to be used by clk providers

As we move towards a cleaner split to have clock providers use clk_hw
for all clock operations, while consumers operate on the (per-user)
struct clk handles, we still have cases where in a clock provider
might want to call into high level clk apis which only operate on a
struct clk handle.
To facilitate such needs, have a clk_hw_get_clk() api which can be
used from within clock providers to get access to struct clk handles.

Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>