MLK-9826 arm: imx6: Add low power idle support for imx6sl

MLK-9826 arm: imx6: Add low power idle support for imx6sl

Enable low power idle for imx6sl. When the busfreq is either
in ultra_low_bus_freq mode or audio_bus_freq_mode, we Can save
more power by reducing the system frequency further in ldle.
At present, Only two idle(WFI and WAIT) are supported.

     WFI        --> normal ARM ilde (first level idle)
     WAIT mode  --> low power idle (second level idle)

When entered WAIT mode, change the DDR, AHB/AXI and ARM clk frequency
as below if the system is in:

  1. ultra_low_bus_freq:
     DDR freq to 1MHz,
     AHB/AXI freq to 3MHz,
     ARM freq to 3MHz.

  2. audio_low_bus_mode:
     DDR freq to 25Hz,
     AHB freq to 8MHz,
     ARM freq to 8MHz.

Anatop can be put in low power mode when all the PLLs are powered down.
We can enable the low power bandgap and disable the rugulator bandgap,
enable the weak 2p5 LDO and disable the 2p5 LDO.

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