common: Move RAM-sizing functions to init.h

These functions relate to memory init so move them into the init
header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/broadcom/bcmns2/northstar2.c b/board/broadcom/bcmns2/northstar2.c
index 292802f..6cbad9c 100644
--- a/board/broadcom/bcmns2/northstar2.c
+++ b/board/broadcom/bcmns2/northstar2.c
@@ -4,6 +4,7 @@
  */
 #include <common.h>
 #include <cpu_func.h>
+#include <init.h>
 #include <asm/system.h>
 #include <asm/armv8/mmu.h>