arm: mvebu: turris_mox: support devices with RAM > 1 GB

In order to support MOX boards with 2 GB or 4 GB RAM, we use the new
Armada-3700 generic code for memory information structures. This is done
by removing dram_init and dram_init_banksize from turris_mox.c, in order
for the generic, weak definitions to be used.

Also for boards with 4 GB RAM it is needed to increase
CONFIG_NR_DRAM_BANKS to 2 in turris_mox_defconfig.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>