Since driver can parse num-cs device property, replace platform data
with this new approach. This pursues the following objectives:

- getting rid of the public header that barely used outside of
  the SPI subsystem (more specifically the SPI PXA2xx drivers)

- making a trampoline for the driver to support non-default number
  of the chip select pins in case the original code is going to be
  converted to Device Tree model

It's not expected to have more users in board files except this one.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240417110334.2671228-3-andriy.shevchenko@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>