MLK-10439: arm: imx: fix system hang after resume back under low busfreq

MLK-10439: arm: imx: fix system hang after resume back under low busfreq

System will be no response after resume back under low busfreq mode. The
root cause of this issue is that when the system is under ultra_low_bus_freq
mode on i.MX6SL, resume process has a low bus mode deamon in background,
cause system enter low bus mode twice, busfreq driver will notify thermal
driver to turn off PLL3 twice, and cause PLL3 usecount incorrect and UART
no message out.

Signed-off-by: Bai Ping <b51503@freescale.com>