common: Move dram_init() declaration to common location

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/arm/include/asm/u-boot-arm.h

@@ -36,7 +36,6 @@ int	arch_early_init_r(void);
 
 /* board/.../... */
 int	board_init(void);
-int	dram_init (void);
 void	dram_init_banksize (void);
 
 /* cpu/.../interrupt.c */