zmx25: Remove dram_init_banksize()

As only one RAM bank is used we can rely on the code from arch/arm/lib/board.c

Cc: Matthias Weisser <weisserm@arcor.de>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>