imx: ahab: fix implicit declaration warning

Fix the following warning:

arch/arm/mach-imx/imx8/ahab.c:105:3: warning: implicit declaration of function ‘flush_dcache_range’ [-Wimplicit-function-declaration]
   flush_dcache_range(s, e);
   ^~~~~~~~~~~~~~~~~~

Include cpu_func.h header which declares the flush_dcache_range()
function.

Signed-off-by: Oliver Graute <oliver.graute@kococonnector.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: uboot-imx <uboot-imx@nxp.com>
diff --git a/arch/arm/mach-imx/imx8/ahab.c b/arch/arm/mach-imx/imx8/ahab.c
index 9df7474..4bb7c46 100644
--- a/arch/arm/mach-imx/imx8/ahab.c
+++ b/arch/arm/mach-imx/imx8/ahab.c
@@ -14,6 +14,7 @@
 #include <asm/arch/sys_proto.h>
 #include <asm/arch/image.h>
 #include <console.h>
+#include <cpu_func.h>
 
 DECLARE_GLOBAL_DATA_PTR;