MLK-9708 arm: imx: add low power idle support for i.mx6sx

MLK-9708 arm: imx: add low power idle support for i.mx6sx

Enable low power idle for i.MX6SX:

   WFI            -> first level idle;
   WAIT mode      -> second level idle;
   Low power idle -> third level idle, only when system is in low bus mode.

In low powe idle mode, below operations will be done:

   ARM power off;
   AHB freq lower to 3MHz;
   PERCLK freq lower to 6MHz;
   MMDC freq lower to 1MHz;

Anatop will be put into low power mode, and regular band-gap will
be off and low power band-gap will be enabled instead.

Also, in low power idle mode, 24MHz XTAL power will be off and 24MHz clk
source will be switched to RC-OSC to save power, this feature is only
enabled on i.MX6SX TO1.2.

This patch is cherry-picked from L3.14.y, it is the latest version, below
conflicts are fixed.

Signed-off-by: Anson Huang <b20788@freescale.com>
Conflicts:
	arch/arm/mach-imx/Makefile
	arch/arm/mach-imx/common.h
	arch/arm/mach-imx/cpuidle-imx6sx.c
	arch/arm/mach-imx/cpuidle.h
	arch/arm/mach-imx/mach-imx6sx.c
	arch/arm/mach-imx/pm-imx6.c