Kirkwood: dram_init is moved to dram.c

For all Kirkwood boards so far dram_init function is duplicated
dram_init function is moved to dram.c and relevant code from all
board specific files removed

If any board needs specific dram init handling than standard one,
then, a macro CONFIG_SYS_BOARD_DRAM_INIT should be defined in
board config header file and the dram_init function can be put
in board specific source file
For ex. keymile boards

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>