spi: fsl-espi: introduce struct fsl_espi

Only few members of struct mpc8xxx_spi are relevant for fsl-espi.
Therefore replace it with a ESPI-specific struct fsl_espi.
Replace variable names mpc8xxx_spi and mspi with espi.

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