[POWERPC] spi: Use fsl_spi instead of mpc83xx_spi

According to booting-without-of.txt, compatible should be "fsl_spi" and
mode "cpu" or "qe" for the fsl SPI controllers.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>