spi:Add Freescale DSPI driver for Vybrid VF610 platform

The serial peripheral interface (SPI) module implemented on Freescale Vybrid
platform provides a synchronous serial bus for communication between Vybrid
and the external peripheral device.
The SPI supports full-duplex, three-wire synchronous transfer, has TX/RX FIFO
with depth of four entries.

This driver is the SPI master mode driver and has been tested on Vybrid
VF610TWR board.

Signed-off-by: Alison Wang <b18965@freescale.com>
Signed-off-by: Chao Fu <b44548@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>