spi: qup: Remove chip select function

This patch removes the chip select function.  Chip select should instead be
supported using GPIOs, defining the DT entry "cs-gpios", and letting the SPI
core assert/deassert the chip select as it sees fit.

The chip select control inside the controller is buggy.  It is supposed to
automatically assert the chip select based on the activity in the controller,
but it is buggy and doesn't work at all.  So instead we elect to use GPIOs.

Signed-off-by: Andy Gross <agross@codeaurora.org>
Signed-off-by: Mark Brown <broonie@linaro.org>