avr32: Move stack_end to arch_global_data

Move this field into arch_global_data and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/avr32/lib/board.c b/arch/avr32/lib/board.c
index e3287c4..d3c8cb7 100644
--- a/arch/avr32/lib/board.c
+++ b/arch/avr32/lib/board.c
@@ -231,7 +231,7 @@
 
 	/* And finally, a new, bigger stack. */
 	new_sp = (unsigned long *)addr;
-	gd->stack_end = addr;
+	gd->arch.stack_end = addr;
 	*(--new_sp) = 0;
 	*(--new_sp) = 0;