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/arch/arm/mach-tegra/board2.c b/arch/arm/mach-tegra/board2.c
index 07f54f0..d3497a2 100644
--- a/arch/arm/mach-tegra/board2.c
+++ b/arch/arm/mach-tegra/board2.c
@@ -8,6 +8,7 @@
 #include <dm.h>
 #include <env.h>
 #include <errno.h>
+#include <init.h>
 #include <ns16550.h>
 #include <usb.h>
 #include <asm/io.h>