spi: ep93xx: Convert to use CS GPIO descriptors

This converts the EP93xx SPI master driver to use GPIO
descriptors for chip select handling.

EP93xx was using platform data to pass in GPIO lines,
by converting all board files to use GPIO descriptor
tables the core will look up the GPIO lines from the
SPI device in the same manner as for device tree.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>