cpufreq: imx6q: add of_init_opp_table

Add a routine check to see if the platform supplied the OPP table.
Incase there's no OPP table exist, it will try to initialise it.

It's been tested on iMX6SL board where the platform doesn't have
an OPP table.

Signed-off-by: John Tobias <john.tobias.ph@gmail.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>