spi: spi-omap2-mcspi.c: Toggle CS after each word

This patch allows the board code to define SPI devices which needs to
toggle the chip select after every word send. This is needed to get a
better resolution reading e.g. an ADC data stream.
Apart from that, as in the normal code CS is controlled by software,
a transfer is done much faster.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>