clk: remove clk_debugfs_add_file()

No one was using this api call, so remove it.  If it is ever needed in
the future, a "raw" debugfs call can be used.

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>