arm: mvebu: dram.c: Rework dram_init() and dram_init_banksize()

Rework these functions so that dram_init_banksize() does not call
dram_init() again. It only needs to set the banksize values in the
bdinfo struct.

Make sure to also clip the size of the last bank if it exceeds the
maximum allowed value of 3 GiB (0xc000.0000). Otherwise other
address windows (e.g. PCIe) will overlap with this memory window.

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