arm: mvebu/armada100: dram.c: Remove CONFIG_SYS_BOARD_DRAM_INIT

CONFIG_SYS_BOARD_DRAM_INIT is not defined anywhere. So lets get rid
of all references here.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>