[POWERPC] spi: mode should be "cpu-qe" instead of "qe"

Mode should be "cpu-qe" for QE in CPU mode. "qe" should be reserved
for native QE mode.

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