MLK-10271 cpufreq: imx6: improve busfreq operation when wehn setpoint lower than...

MLK-10271 cpufreq: imx6: improve busfreq operation when wehn setpoint lower than 396MHz

for i.MX6SX, according to the latest datasheet, added a 198MHz setpoint in cpufreq driver.
The 198MHz setpoint is NOT enough to support playing mp3,the system will stay at a higher
setpoint and high_bus_mode. So when having a setpoint lower than 396MHz, make sure when
the cpufreq is at 396MHz or lower, the busfreq is always in low_bus_mode to save more power.

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