mx6: fix warning in platinum board

Fix warning due to missing prototype for writel

Signed-off-by: Stefano Babic <sbabic@denx.de>
Acked-by: Stefan Roese <sr@denx.de>

board/barco/platinum/platinum.h

@@ -9,6 +9,7 @@
 
 #include <miiphy.h>
 #include <asm/arch/crm_regs.h>
+#include <asm/io.h>
 
 /* Defines */