spi: Add support for specifying 3-wire mode via device tree

This patch allows to specify that a SPI device is connected in 3-wire mode via
device tree.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>