arm64: mvebu: Replace board specific with generic memory bank decoding

The dram_init and dram_init_banksize functions were using a board
specific implementation for decoding the memory banks from the fdt.
This change makes the dram_init* functions use a generic implementation
of decoding and populating memory bank and size data.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Nathan Rossi <nathan@nathanrossi.com>
Cc: Nadav Haklai <nadavh@marvell.com>
Cc: Kostya Porotchkin <kostap@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>