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>

lib/efi_loader/efi_memory.c

@@ -7,6 +7,7 @@
 
 #include <common.h>
 #include <efi_loader.h>
+#include <init.h>
 #include <malloc.h>
 #include <mapmem.h>
 #include <watchdog.h>