am43xx: Update EMIF DDR3 Configuration for AM43x GP

* Boot failures have been discovered due to a combination of routing issues and
  non optimal ddr3 timings in the EMIF
* Since ddr3 timings are different after significant board layout changes
  different timings are required for alpha, beta and production boards.

Signed-off-by: Franklin S. Cooper Jr <fcooper@ti.com>