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>
diff --git a/drivers/video/sunxi/sunxi_display.c b/drivers/video/sunxi/sunxi_display.c
index b1aa4b2..31f0aa7 100644
--- a/drivers/video/sunxi/sunxi_display.c
+++ b/drivers/video/sunxi/sunxi_display.c
@@ -9,6 +9,7 @@
 #include <common.h>
 #include <cpu_func.h>
 #include <efi_loader.h>
+#include <init.h>
 #include <time.h>
 
 #include <asm/arch/clock.h>