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>