arm64: dts: sparx5: Add SPI controller and associated mmio-mux

This adds a SPI controller to the Microchip Sparx5 SoC, as well as the
mmio-mux that is required to select the right SPI interface for a
given SPI device.

Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Link: https://lore.kernel.org/r/20200824203010.2033-4-lars.povlsen@microchip.com