MLK-11252-02 cpufreq: imx: add dc regulator in cpufreq driver

In some i.MX6 board (i.MX6UL EVK), it will have a additional
GPIO controlled DC-DC regulator. we need to change this regulator's
voltage According the maximum frequency that CPU can run at.

At present, we only need to set the voltage to minimum one, we need
to set the voltage to the maximum before suspend, resume back to the
minimum voltage after suspend.

Signed-off-by: Bai Ping <b51503@freescale.com>