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

The iomux header is included on sys_proto.h so to avoid SoC specific
header inclusion.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Marek Vasut <marex@denx.de>

arch/arm/cpu/arm926ejs/mxs/spl_boot.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 <asm/arch/sys_proto.h>
 #include <asm/gpio.h>