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>

board/raspberrypi/rpi/rpi.c

@@ -10,6 +10,7 @@
 #include <efi_loader.h>
 #include <fdt_support.h>
 #include <fdt_simplefb.h>
+#include <init.h>
 #include <lcd.h>
 #include <memalign.h>
 #include <mmc.h>