cpufreq: imx6q: fix speed grading regression on i.MX6 QuadPlus

The commit moving the speed grading check to the cpufreq driver introduced
some additional checks, so the OPP disable is only attempted on SoCs where
those OPPs are present. The compatible checks are missing the QuadPlus
compatible, so invalid OPPs are not correctly disabled there.

Move both checks to a single condition, so we don't need to sprinkle even
more calls to of_machine_is_compatible().

Fixes: 2b3d58a3 (cpufreq: imx6q: Move speed grading check to cpufreq driver)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>