spi: spi-ep93xx: simplify GPIO chip selects

This driver requires a GPIO line to be used for the chip select of
each SPI device.

Remove the ep93xx_spi_chip_ops definition from the platform data
and use the spi core GPIO handling for the chip selects.

Fix all the ep93xx platforms that use this driver and remove the
old Documentation.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Mark Brown <broonie@kernel.org>