cpufreq: imx6: remove pu regulator dependency for SOCs with no PU regulator

PU regulator is not a necessary regulator for cpufreq, not all
i.MX6 SoCs have PU regulator, only if SOC has PU regulator, then its
voltage must be equal to SOC regulator, so remove the dependency
to support i.MX6SX which has no PU regulator.

Signed-off-by: Anson Huang <b20788@freescale.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>