ARM: vf610: Move DDR3 initialization to imx-common

In order to avoid code duplication, move the DDR3 initialization to the
common place under imx-common. Currently ROW_DIFF and COL_DIFF can be
chosen from the board file. The JEDEC timings are specified using a
common ddr3_jedec_timings structure.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>