add SPI struct for master/slave bit

[DmitriLyalikov]

I am working on a downstream implementation for a spi slave, and would like to ask, is there a reason that the Spi Mode struct does not include a master or slave type as well? I would like to add this, and I am working actually on the driver for the rp2040 hal. What is the philosophy for not doing this at the generic hal level?

/// SPI mode
#[derive(Clone, Copy, PartialEq, Eq)]
pub struct Mode {
/// Clock polarity
pub polarity: Polarity,
/// Clock phase
pub phase: Phase,
}

[Dirbaio]

The current SPI traits are intentionally for SPI master mode only. The reason is for slave mode the API should be different, so it should be a different trait, not a "mode" bit in the existing trait. See previous discussion at #396