am335x: Enable DDR PHY dynamic power down bit for DDR3 boards

Enable DDR PHY dynamic power down bit, which enables
powering down the IO receiver when not performing read.

This also helps in reducing overall power consumption in
low power states (suspend/standby).

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Satyanarayana, Sandhya <sandhya.satyanarayana@ti.com>
Cc: Tom Rini <trini@ti.com>
Reviewed-by: Tom Rini <trini@ti.com>