ARM: imx53: Set DDR pins to high impedance when in suspend to RAM.

In order to save power the DDR pins should be put into high
impedance when in suspend to RAM.

This requires manually requesting self refresh (rather than using the
automatic mode implemented by the CCM / ESDCTL), followed by
reconfiguring the IOMUXC.

Of course the code to do this cannot itself run from DDR so the
code is copied to and executed from internal memory.

In my tests using a custom i.MX53 board with LPDDR2 RAM
this reduced the suspend power consumption from 200mW to 60mW.

Signed-off-by: Martin Fuzzey <mfuzzey@parkeon.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>