clk: Add support for setting clk_rate via debugfs

For testing, it is useful to be able to specify a clock rate manually.
As this is a dangerous feature, it is not enabled by default.
Users need to modify the source directly and #define
CLOCK_ALLOW_WRITE_DEBUGFS.

This follows the spirit of commit 09c6ecd3

 ("regmap: Add support
for writing to regmap registers via debugfs").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lkml.kernel.org/r/20190828132306.19012-1-geert+renesas@glider.be
Suggested-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>