spi: add mraa_spi_init_raw

This lets you access a spidev device directly without any checking in the style
of mraa_i2c_init_raw

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

api/mraa/spi.h

@@ -70,6 +70,15 @@ typedef struct _spi* mraa_spi_context;
  */
 mraa_spi_context mraa_spi_init(int bus);
 
+/**
+ * Initialise SPI_context without any board configuration, selects a bus and a mux.
+ *
+ * @param bus Bus to use as listed by spidev
+ * @param cs Chip select to use as listed in spidev
+ * @return Spi context or NULL
+ */
+mraa_spi_context mraa_spi_init_raw(unsigned int bus, unsigned int cs);
+
 /**
  * Set the SPI device mode. see spidev 0-3.
  *