spi: dw: Add Microchip Sparx5 support

spi: dw: Add Microchip Sparx5 support

This adds SPI support for the Sparx5 SoC, which is using the MMIO
Designware SPI controller.

The Sparx5 differs from the Ocelot version in these areas:

 * The CS override is controlled by a new set of registers for
   this purpose.

 * The Sparx5 SPI controller has the RX sample delay register, and it
   must be configured for the (SPI NAND) device on SPI2.

 * The Sparx5 SPI controller has 2 different SPI bus interfaces on the
   same controller (don't ask...). The "spi-mux" driver should be used
   in conjunction with the SPI driver to select the appropriate bus.

Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Link: https://lore.kernel.org/r/20200824203010.2033-3-lars.povlsen@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>