want to be able to specify device for `humility spi`

humility i2c allows me to specify a specific device in the app.toml which is great. It would be nice to have this for SPI and have it take care of chip selects. In particular, this comes to a head when we're dealing with the FPGAs that are on the Sidecar front I/O board. While they are both on the SPI1 peripheral, there are multiple different pins that are being used for chip selects and an internal mux to boot. For example, in the app.toml we see:

[config.spi.spi1.mux_options.port_adg]
outputs = [
    {port = "A", pins = [5], af = 5}, # FRONT_IO_SCK
    {port = "D", pins = [7], af = 5}, # FRONT_IO_COPI
]
input = {port = "G", pin = 9, af = 5} # FRONT_IO_CIPO

[config.spi.spi1.devices.ecp5_front_io_fpga]
mux = "port_adg"
cs = [{port = "G", pin = 10}] # FRONT_IO_CS0

[config.spi.spi1.devices.ecp5_front_io_user_design]
mux = "port_adg"
cs = [{port = "A", pin = 15}] # FRONT_IO_CS1

If you try to just use a plain spi1 it's not clear what it's talking to and doesn't necessarily work.

Start with the humility spi and humility i2c entry points, then inspect the app.toml configuration under config.spi.spi1.devices. Add a way for humility spi to select ecp5_front_io_fpga or ecp5_front_io_user_design and use its configured chip select; verify that selecting a device works when multiple chip selects share SPI1.