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>

drivers/video/fsl_dcu_fb.c

@@ -6,6 +6,7 @@
  * FSL DCU Framebuffer driver
  */
 
+#include <init.h>
 #include <asm/io.h>
 #include <common.h>
 #include <dm.h>

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>