cpufreq: imx6q-cpufreq driver is reused on i.MX6 series SoCs

The imx6q-cpufreq driver nowadays is not only running on imx6q but also
other i.MX6 series SoCs like imx6dl and imx6sl.  Update Kconfig prompt
and help text to make it clear to users.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>