serial: mxs-auart: Remove <asm/cacheflush.h>

There is nothing in the driver that uses the definitions
from <asm/cacheflush.h>.

Remove the unused header file inclusion.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20210118152154.1644569-2-festevam@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>