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>

common/board_f.c

@@ -20,6 +20,7 @@
 #include <fdtdec.h>
 #include <fs.h>
 #include <i2c.h>
+#include <init.h>
 #include <initcall.h>
 #include <lcd.h>
 #include <malloc.h>