spi: fsl-espi: align register access with other drivers

Change register access to the method used in other drivers too.
- use register names as in the chip spec for constants
- avoid hard to read statements like
  __be32 __iomem *espi_mode = &reg_base->mode
- get rid of old powerpc-specific functions like in_8

In addition annotate reg_base in struct mpc8xxx_spi as __iomem.

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