MLK-9996 arm: imx6: Correct the AHB clock in low_bus_freq_mode

When the busfreq is in audio_bus_freq_mode, the AHB bus is at 8MHz,
in low_bus_freq_mode, the AHB needs to run at 24MHz. So when switching
from audio_bus_freq_mode to low_bus_freq_mode, make sure the AHB is at
24MHz in low_bus_freq_mode.

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