ENGR00210469 MX6q - Fix suspend/resume failure.

ENGR00210469 MX6q - Fix suspend/resume failure.

Sourcing AXI_CLK from PLL3_PFD_540M causes the system to
hang on resuming from STOP mode.
The main issue is that PFDs may sometimes hang/freeze
when their parent PLLs are powered on and then relocked
when exiting from STOP mode. To avoid this, PFDs must
be disabled before entering STOP and enabled after resume.

The fix is to move axi_clk to periph_clk before system
enters STOP and then restore it back to PLL3_PFD_540M
after resume.

Signed-off-by: Ranjani Vaidyanathan <ra5478@freescale.com>