stm32/spi: Add implementation of low-level SPI protocol.

Can be used, for example, to configure external SPI flash using a hardware
SPI interface (code to be put in a board's bdev.c file):

    STATIC const spi_proto_cfg_t hard_spi_bus = {
        .spi = &spi_obj[5],
        .baudrate = 10000000,
        .polarity = 0,
        .phase = 0,
        .bits = 8,
        .firstbit = SPI_FIRSTBIT_MSB,
    };

    STATIC mp_spiflash_cache_t spi_bdev_cache;

    const mp_spiflash_config_t spiflash_config = {
        .bus_kind = MP_SPIFLASH_BUS_SPI,
        .bus.u_spi.cs = pin_A0,
        .bus.u_spi.data = (void*)&hard_spi_bus,
        .bus.u_spi.proto = &spi_proto,
        .cache = &spi_bdev_cache,
    };

    spi_bdev_t spi_bdev;

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