i2c_spi_buses: add support for multiple instances of the same device on a bus

This also simplifies the API a bit, since we anyway have to change the
drivers to pass additional information (the bus device index).

The orientation flag is merged with the rotation.

platforms/common/include/px4_platform_common/spi.h

@@ -158,11 +158,13 @@ public:
 
 	bool external() const { return px4_spi_bus_external(bus()); }
 
+	int busDeviceIndex() const { return _bus_device_index; }
+
 private:
 	const FilterType _filter;
 	const uint16_t _devid_driver_index;
 	const int _bus;
-	int _index{-1};
-	int _external_bus_counter{0};
-	int _bus_device_index{0};
+	int _index{0};
+	int _external_bus_counter{1};
+	int _bus_device_index{-1};
 };