cpufreq: imx6q: read OCOTP through nvmem for imx6q

Directly accessing OCOTP registers should be avoided, because
it could not handle OCOTP clks and could not handle defer
proper. With nvmem API, it is safe to access OCOTP registers.

To make sure old dtb could work, the original code still kept.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>