MLK-10018-02 thermal: imx: notify thermal driver in low_bus_freq_mode

As thermal sensor alarm function needs PLL3 to be always on, but low power
idle needs all PLLs to be off, they are exclusive. Low power idle is only enabled
when system staying at low bus mode which means the overall system power consumption
is NOT high, thermal alarm function can be disabled in this mode to allow low power
idle to be entered, and thermal sensor will still use polling mechanism to monitor
the system temperature. Add busfreq notify to achieve this goal.

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