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/lib/init_helpers.c

@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <init.h>
 #include <linux/errno.h>
 #include <asm/mtrr.h>