ENGR00162318: MX6: Donot power down PLL when PLL is disabled

ENGR00162318: MX6: Donot power down PLL when PLL is disabled

Relocking of PLL sometimes takes longer than 1msec when PLL is
enabled after a power down. The kernel panics if the PLL is not locked
in 1ms. Max time expected by HW is only 500us.
Workaround is to disable the output of the PLL and set it in bypass
mode when its output is not being used.

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