mxs: Remove not required include of iomux-mx28.h

The iomux-mx28.h include is not required on spl_mem_init.c so it has
been droped.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>

arch/arm/cpu/arm926ejs/mxs/spl_mem_init.c

@@ -26,7 +26,6 @@
 #include <common.h>
 #include <config.h>
 #include <asm/io.h>
-#include <asm/arch/iomux-mx28.h>
 #include <asm/arch/imx-regs.h>
 
 #include "mxs_init.h"