ENGR00156389: turn off child clocks before reconfigure perclk_root

In addition to ensuring that PERCLK remains at least 2.5 times slower
than the AHB clock, certain steps need to be followed to ensure robust
operation of PERCLK when reconfiguring the PERCLK clock source.

To properly configure the PERCLK clock source, the following steps are
required:
1.In the CCGR registers, gate the clocks to all PERCLK-dependent
  modules.
2.Select the desired input clock for the PERCLK root clock (to be either
  source from the peripherals main source clock or the
  lp_apm clock source). Refer to the CMCBR register,
  perclk_lp_apm_sel bit.
3.Configure the perclk_pred1, perclk_pred2, and perclk_podf dividers
  to the desired setting.  Refer to the CBCDR register for details.
  4.In the CCGR registers, enable the desired clocks for the
  PERCLK-dependent module clocks.

Certain steps are required to reconfigure perclk_root.
If don't follow these steps, GPT timer may stop and the kernel stops
at " "Calibrating delay loop".

Signed-off-by: Terry Lv <r65388@freescale.com>

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