cpufreq: imx: Select NVMEM_IMX_OCOTP

When probing cpufreq for iMX6 the values in the efuse needs to be
read which requires NVMEM_IMX_OCOTP. If this option is not enabled,
the probe will be deferred forever and cpufreq won't be available.

This patch forces the selection of the required configuration option.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>