clkdev: Add clk_hw based registration APIs

Now that we have a clk registration API that doesn't return
struct clks, we need to have some way to hand out struct clks via
the clk_get() APIs that doesn't involve associating struct clk
pointers with a struct clk_lookup. Luckily, clkdev already
operates on struct clk_hw pointers, except for the registration
facing APIs where it converts struct clk pointers into struct
clk_hw pointers almost immediately.

Let's add clk_hw based registration APIs so that we can skip the
conversion step and provide a way for clk provider drivers to
operate exclusively on clk_hw structs. This way we clearly
split the API between consumers and providers.

Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>