fsl_qspi: Pet the watchdog while reading/writing

When reading a large blob. e.g. a linux kernel (several MiBs) a watchdog
timeout might occur meanwhile. So pet the watchdog while operating on
the flash.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Reviewed-by: York Sun <yorksun@freescale.com>