ENGR00154917 [MX6]Enable MMDC low-power mode

ENGR00154917 [MX6]Enable MMDC low-power mode

1. Set MMDC pad ctrl to low-power mode when dormant;
2. DRAM_RESET can't be changed due to hardware design;
3. GPR_CTLDS should be changed to lower the MMDC IO
power to 0mA, but it needs hardware change, will add it
in next hardware version after we figure out how to
change the hardware.

Current MMDC data in dormant is:
IO: 28mA@1.5V;
DDR: 35mA@1.5V.

4. Change the suspend code to run in iRAM;

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