MLK-11281-6 ARM: imx: add lpsr support for imx7d

Add LPSR mode support if dtb contains "fsl,enable-lpsr" property,
when echo mem to make system enter DSM mode, whole SOC will be
powered down except LPSR, SNVS domain and DDR chip's power,
that means all modules in SOC domain will be powered down,
including ccm, iomuxc, gpc....

So, all drivers need to restore their iomux settings and
clk settings after resume;

When system enters LPSR mode, can be waked up by long press
ON/OFF button or using RTC alarm.

In LPSR mode resume, ROM will read the entry point in LPSR
register, make DRAM exit retention mode and jump to DRAM
to resume kernel immediately, so before entering LPSR mode,
we need to set the resume entry correctly in LPSR register
and clear it after resume.

Signed-off-by: Anson Huang <b20788@freescale.com>