MLK-10879-03 cpufreq: imx: add cpufreq support for i.mx6ul.

Add cpufreq driver for i.MX6UL for DVFS. According to the design team, the ARM core can run
at setpoints 528MHz, 396MHz and 198MHz. This SOC adds a secondary_clk_sel(pll2/pll2_pfd2)
as the clock source of step_clk. In normal situation, just use the step_clk as the ARM
clk source, the pll1_sys is only used when change the cpu frequency.

Conflicts:
	arch/arm/mach-imx/mach-imx6ul.c
	drivers/cpufreq/imx6q-cpufreq.c

Signed-off-by: Bai Ping <b51503@freescale.com>
(cherry picked from commit f6528f7922c2ba1cacabf93eecb9301fb20d459e)