ARM: AM335x: Enable DDR dynamic IO power down

ARM: AM335x: Enable DDR dynamic IO power down

This patch enables dynamically powering down the
IO receiver when not performing a read on boards using DDR3.
This optimizes both active and standby power consumption.
This bit is not set on EVM SK and EVM 1.5 and later boards.
Setting the same.

This has been tested on PG2.0 EVM1.5, EVM1.2, EVM-SK, BBB.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Satyanarayana, Sandhya <sandhya.satyanarayana@ti.com>