cpufreq: imx6q: Fixes unwanted cpu overclocking on i.MX6ULL

cpufreq: imx6q: Fixes unwanted cpu overclocking on i.MX6ULL

imx6ul_opp_check_speed_grading is called for both i.MX6UL and i.MX6ULL.
Since the i.MX6ULL was introduced to a separate ocotp compatible node
later, it is possible that the i.MX6ULL has also dtbs with
"fsl,imx6ull-ocotp". On a system without nvmem-cell speed grade a
missing check on this node causes a driver fail without considering
the cpu speed grade.

This patch prevents unwanted cpu overclocking on i.MX6ULL with compatible
node "fsl,imx6ull-ocotp" in old dtbs without nvmem-cell speed grade.

Fixes: 2733fb0d0699 ("cpufreq: imx6q: read OCOTP through nvmem for imx6ul/imx6ull")
Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>