missing API : spi_get_instance

In hardware/i2c, there is a i2c_get_instance function but no counterpart in hardware/spi : spi_get_instance.

something like this : 

```
static spi_inst_t* spi_get_instance( uint num ) {
    invalid_params_if( HARDWARE_SPI, num >= NUM_SPIS );
	return SPI_INSTANCE( num );
}
```