ENGR00225894 MX6SL-Improve system IDLE power numbers

ENGR00225894 MX6SL-Improve system IDLE power numbers

Add the following power optimizations when all PLLs
are either disabled or in bypass:
1. Disable 2P5 in system IDLE mode and enable weak 2P5.
2. Set OSC bias current to -37.5% just before the WFI instruction
and set it back to 0% after WFI.
3. Enable the low power bandgap and power down the regular
bandgap.

Also lower AHB and AXI to 3MHz in this mode.

Signed-off-by: Ranjani Vaidyanathan <ra5478@freescale.com>