serial: mfd: Add nmi_touch_watchdog() into the console write function

This is following what 8250 driver is doing in console write function,
to avoid the hardware lockup case.

v2: incldudes the <linux/nmi.h>

Signed-off-by: Feng Tang <feng.tang@intel.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>