cpufreq: imx6q: Fix imx6sx low frequency support

This patch contains the minimal changes required to support imx6sx OPP
of 198 Mhz. Without this patch cpufreq still reports success but the
frequency is not changed, the "arm" clock will still be at 396000000 in
clk_summary.

In order to do this PLL1 needs to be still kept enabled while changing
the ARM clock. This is a hardware requirement: when ARM_PODF is changed
in CCM we need to check the busy bit of CCM_CDHIPR bit 16 arm_podf_busy,
and this bit is sync with PLL1 clock, so if PLL1 NOT enabled, this
bit will never get clear.

Keep pll1_sys explicitly enabled until after the rate is change to deal
with this. Otherwise from the clk framework perspective pll1_sys is
unused and gets turned off.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>