enhance(spi): implement spi for embedded-hal-async

[bdelwood]

Essentially this PR, but for SPI.

Tested on a ch32v003 with this small example and checked with a logic analyzer:

#![no_std]
#![no_main]

use ch32_hal as hal;

use embassy_executor::Spawner;
use embassy_time::{Delay, Timer};
use embedded_hal_async::spi::SpiDevice;
use embedded_hal_bus::spi::ExclusiveDevice;
use hal::gpio::{Level, Output};
use hal::prelude::*;
use hal::spi::Spi;
use panic_halt as _;
use qingke_rt;

#[embassy_executor::main(entry = "qingke_rt::entry")]
async fn main(_spawner: Spawner) {
    let config = hal::Config::default();
    let p = hal::init(config);

    let sck = p.PC5;
    let sda = p.PC6;
    let dma_spi = p.DMA1_CH3;

    let mut led: Output<'_> = Output::new(p.PC4, Level::Low, Default::default());
    let cs = Output::new(p.PC7, Level::High, Default::default());

    let mut spi_config = hal::spi::Config::default();
    spi_config.frequency = Hertz::mhz(1);

    let spi_bus = Spi::new_txonly::<0>(p.SPI1, sck, sda, dma_spi, spi_config);

    let mut spi = ExclusiveDevice::new(spi_bus, cs, Delay).unwrap();

    let buf: [u8; 2] = [0xDE, 0xAD];

    loop {
        spi.write(&buf).await.unwrap();

        Timer::after_millis(100).await;
        led.toggle();
    }
}