spi: spi-microchip-core: Add support for GPIO based CS

spi: spi-microchip-core: Add support for GPIO based CS

The SPI "hard" controller within the PolarFire SoC is capable of
handling eight CS lines, but only one CS line is wired. Therefore, use
GPIO descriptors to configure additional CS lines.

Signed-off-by: Prajna Rajendra Kumar <prajna.rajendrakumar@microchip.com>
Link: https://msgid.link/r/20240514104508.938448-4-prajna.rajendrakumar@microchip.com
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>