Spi: Constructor with chip select added in cpp header

Signed-off-by: Steven Müllener <steven.muellener@sius.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

api/mraa/spi.hpp

@@ -70,6 +70,15 @@ class Spi
         }
     }
 
+    Spi(int bus, int cs)
+    {
+        m_spi = mraa_spi_init_raw(bus, cs);
+
+        if (m_spi == NULL) {
+            throw std::invalid_argument("Error initialising SPI bus");
+        }
+    }
+
     /**
      * Closes spi bus
      */