cpufreq: imx6: use regmap to read ocotp register

Reading the ocotp register directly is unsafe and will cause the system
to hang if its clock is not turned on in CCM. The regmap interface has
clk enabled, which can solve this problem.

Signed-off-by: tianyu2 <tianyu2@kernelsoft.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>