clk: fixed-rate: Add hw based registration APIs

clk: fixed-rate: Add hw based registration APIs

Add registration APIs in the clk fixed-rate code to return struct
clk_hw pointers instead of struct clk pointers. This way we hide
the struct clk pointer from providers unless they need to use
consumer facing APIs.

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