spi, mpc8xx: Add support for chipselect via GPIO and fixups

This patch fixes the mpc8xx SPI driver:
- A stub callbacks for mode and speed,
- Use chip selects defined as GPIOs,
- Write proper value to disable relocation, other it fails on mpc885,
- Don't modify ports setup, ports can be different from one board to
another and are already set by board_early_init_r().

This patch was originally written by Charles Frey who's
email address is not valid anymore as he left the company.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: FRANJOU Stephane <stephane.franjou@csgroup.eu>