hw/ssi: Add Ibex SPI device model

hw/ssi: Add Ibex SPI device model

Adds the SPI_HOST device model for ibex. The device specification is as per
[1]. The model has been tested on opentitan with spi_host unit tests
written for TockOS.

[1] https://docs.opentitan.org/hw/ip/spi_host/doc/

Signed-off-by: Wilfred Mallawa <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20220303045426[email protected]>
Signed-off-by: Alistair Francis <[email protected]>