spi: add support for microchip fpga spi controllers

Add a driver for Microchip FPGA SPI controllers, specifically
supporting the "hard" controllers on PolarFire SoC.

Co-developed-by: Daire McNamara <daire.mcnamara@microchip.com>
Signed-off-by: Daire McNamara <daire.mcnamara@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220607073833.2331539-2-conor.dooley@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>