omap4: calculate EMIF register values

Calculate EMIF register values based on AC timing parameters
from the SDRAM datasheet and the DDR frequency rather than
using the hard-coded values.

For a new board the user doen't have to go through the tedious
process of calculating the register values. Instead, just
provide the AC timings from the device data sheet as input
and the driver will automatically calculate the register values.

Signed-off-by: Aneesh V <aneesh@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>