cpufreq: imx: update the clock switch flow to support imx6ul

For i.MX6UL, the clock switch flow is slightly different from
other i.MX6 SOCs. It has a 'secondary_sel' clk that will be used
when the CPU freq is higher than 396MHz. So the clock switch flow in
'set_target' callback need to update to support i.MX6UL in the common
i.MX6 SOC cpufreq driver.

Signed-off-by: Bai Ping <b51503@freescale.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>