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>

arch/arm/mach-mvebu/arm64-common.c

@@ -6,6 +6,7 @@
 #include <common.h>
 #include <dm.h>
 #include <fdtdec.h>
+#include <init.h>
 #include <linux/libfdt.h>
 #include <linux/sizes.h>
 #include <pci.h>

arch/arm/mach-rockchip/sdram.c

@@ -5,6 +5,7 @@
 
 #include <common.h>
 #include <dm.h>
+#include <init.h>
 #include <ram.h>
 #include <asm/io.h>
 #include <asm/arch-rockchip/sdram.h>

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>