cpufreq: arm: Kconfig: Add i.MX7 to supported SoC for ARM_IMX_CPUFREQ_DT

Since commit a5a9dffc ("ARM: imx: Switch imx7d to imx-cpufreq-dt
for speed-grading") i.MX7 uses this driver as well. Add it to the
description text.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>