spi: davinci: fix to support more than 2 chip selects

Currently, the driver defines SPI_MAX_CHIPSELECT as 2 and
use per device array bytes_per_word based on this. This breaks
if num_chipselect per device is greater than 2. This patch
fix this and allocate memory for this array based on
num_chipselect.

It's preparation patch to enable GPIO CS feature for
Davinci SPI.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>