spi: davinci: Remove chip select GPIO pdata

The DaVinci SPI can use either:
- Internal chip selects (inside the SPI host)
- External chip selects (using GPIO)
- External chip selects passed in pdata

The last way of passing external chip selects through
platform data is not used in the kernel. Delete it to make
the code simpler when refactoring GPIO.

Cc: Sekhar Nori <nsekhar@ti.com>
Cc: Kevin Hilman <khilman@kernel.org>
Cc: Michele Dionisio <michele.dionisio@gmail.com>
Cc: Frode Isaksen <fisaksen@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>