Add Sonata IO getting started guide

[GregAC]

Certainly room for improvement here but I'm keen to have something available with or very shortly after (i.e. today) the v1.0 release so people have a chance of working out how to use the IO.

There's a bunch of links to CHERIoT RTOS and Sonata System here. I think these should be linked to a specific commit/tag so they definitely correspond to v1.0. I haven't done that yet as things were still changing whilst I was writing this.

[GregAC]

Couple of things still to do here:

• dig out more ADC spec details (in particular voltage range!)
• a short section explaining how you instantiate a driver and point it to a particular device
• add usbdev to the sonata-prerelease board definition.

[marnovandermaas]

This looks reasonable to me. I would like Alex's review before merging this.

[AlexJones0]

Looks good overall in terms of content but needs some fixes first.

[GregAC]

Thanks for the reviews @AlexJones0 and @marnovandermaas I believe I have addressed all the comments.

The latest version also includes small new sections on the SPI CS lines and usage of `MMIO_CAPABILITY' to get access to a driver instance.

[AlexJones0]

All looks good to me, thanks for the work on this @GregAC. Just 1 minor comment that was not quite fully addressed and this should be good to go!

[AlexJones0]

Suggested change

	pinmux->output_pin_select(SonataPinmux::OutputPin::pmod0_1, 2);
	pinmux->block_input_select(SonataPinmux::BlockInput::spi_1_cipo, 3);
	pinmux->output_pin_select(SonataPinmux::OutputPin::pmod0_2, 2);
	pinmux->output_pin_select(SonataPinmux::OutputPin::pmod0_4, 2);
	pinmux.output_pin_select(SonataPinmux::OutputPin::pmod0_1, 2);
	pinmux.block_input_select(SonataPinmux::BlockInput::spi_1_cipo, 3);
	pinmux.output_pin_select(SonataPinmux::OutputPin::pmod0_2, 2);
	pinmux.output_pin_select(SonataPinmux::OutputPin::pmod0_4, 2);

Still not quite correct - pinmux is now a SonataPinmux object and not a pointer, so -> usage should be replaced with .

[GregAC]

Ah whoops.

Maybe I should do something radical like actually compile my code snippet and check it works...

[GregAC]

Have demonstrated this working in a test program. May add that as an example (demonstrates using the pmod SPI) but that's a job for Monday!