SPI Slave support

[TheMotonaut]

I've been looking into implementing a SPI slave into the STM32H747 but have not found any documentation on it. Is there a way to implement it in the current version of the crate?

[twantonie]

I've added SPI slave support in my fork of the HAL for the h730. https://github.com/twantonie/stm32h7xx-hal/tree/chronoptics_changes

[TheMotonaut]

Nice! I will have a go at it