common: Move board_get_usable_ram_top() out of common.h

Move this function into init.h which seems to be designed for this sort
of thing. Also update the header to declare struct global_data so that it
can be included without global_data.h being needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

arch/x86/cpu/slimbootloader/sdram.c

@@ -4,6 +4,7 @@
  */
 
 #include <common.h>
+#include <init.h>
 #include <linux/sizes.h>
 #include <asm/e820.h>
 #include <asm/arch/slimbootloader.h>