common: Move dram_init() declaration to common location

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

include/common.h

@@ -183,6 +183,7 @@ typedef void (interrupt_handler_t)(void *);
 /*
  * Function Prototypes
  */
+int dram_init(void);
 
 void	hang		(void) __attribute__ ((noreturn));