spi: fsl-espi: migrate relevant parts of mpc8xxx_spi_probe and of_mpc8xxx_spi_probe

Very little of the library functions mpc8xxx_spi_probe and
of_mpc8xxx_spi_probe is relevant for fsl-espi.

Therefore migrate the relevant parts to fsl-espi (considering
that get_brgfreq() always returns -1 on systems with ESPI)
and remove use of these functions.

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