dt-bindings: Remove outdated cpufreq-dt.txt

The information present in this file is outdated and doesn't serve any
purpose with the current design of the driver.

Remove the outdated file.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
[ Viresh: Rewrite commit log ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>