clk: Update docs after removal of clk-private.h

Currently Documentation/clk.txt describes an obsolete techinique to
statically define struct clk objects.

This capability was removed by b09d6d99("clk: remove clk-private.h")
and is no longer supported. The documentation describing the feature should
be removed.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michael Turquette <mturquette@linaro.org>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>