For consistency reasons, add a function spi_get_baudrate()

There is a setter but no getter. Maybe sometimes required:

```
uint spi_get_baudrate(const spi_inst_t *spi)
{
  uint prescale = spi_get_hw(spi)->cpsr;
  uint postdiv = ((spi_get_hw(spi)->cr0  & SPI_SSPCR0_SCR_BITS) >> SPI_SSPCR0_SCR_LSB) + 1;
  return clock_get_hz(clk_peri) / (prescale * postdiv);
}

```