ARM: imx6: disable bandgap self-bias after boot

The self-bias circuit is used by the bandgap during startup.
Once the bandgap has stabilized, the self-bias circuit should
be disabled for best noise performance of analog blocks.
Also this bit should be disabled before the chip enters STOP mode or
when ever the regular bandgap is disabled.

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