Add DDR3 support into the MPC8xxx DDR driver.

To avoid confusion, the function set_ddr_sdram_mode is renamed
set_ddr2_sdram_mode.

Checking for errors is simplified in the DDR2 DIMM parameters
computation to be consistent with DDR3.

This code is derived from the files found in directory drivers/ddr/fsl
from U-Boot version git-be937b5.

Signed-off-by: Renaud Barbier <renaud.barbier@ge.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>