spi: fsl-espi: add support for dual output read mode

This patch adds support for dual output read mode.

It was successfully tested on a P1014-based device with S25FL128S
SPINOR flash. With 50MHz SPI clock the read rate is 11MByte/s.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>