MLK-10018-01 arm: imx: Implement busfreq notifier calls for busfreq

Implement busfreq notifier calls used when busfreq entering low_bus_freq_mode.
When the system lower the bus frequency, some modules can be affected by bus
frequency change. Adding notifier call chains that allow driver affected by bus
frequency can be notified before and after low_bus_freq_mode.

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