cpufreq: imx6q: read OCOTP through nvmem for imx6ul/imx6ull

cpufreq: imx6q: read OCOTP through nvmem for imx6ul/imx6ull

On i.MX6UL/i.MX6ULL, accessing OCOTP directly is wrong because
the ocotp clock needs to be enabled first. Add support for reading
OCOTP through the nvmem API, and keep the old method there to
support old dtb.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Arulpandiyan Vadivel <arulpandiyan_vadivel@mentor.com>
(cherry picked from commit ef800b8b7eed1363b662477b134b489f44410530)