spi: mpc8xx: Migrate to DM_SPI

Drop non-dm code and migrate into DM_SPI.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[jagan: Move config menu in DM_SPI area]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>