AP_HAL_Linux:SPIDevice: Add functions to get possible spi registered device

libraries/AP_HAL_Linux/SPIDevice.cpp

@@ -454,6 +454,15 @@ SPIDeviceManager::get_device(const char *name)
     return dev;
 }
 
+uint8_t SPIDeviceManager::get_count() {
+   return _n_device_desc;
+}
+
+const char* SPIDeviceManager::get_device_name(uint8_t idx)
+{
+    return _device[idx].name;
+}
+
 /* Create a new device increasing the bus reference */
 AP_HAL::OwnPtr<AP_HAL::SPIDevice>
 SPIDeviceManager::_create_device(SPIBus &b, SPIDesc &desc) const

libraries/AP_HAL_Linux/SPIDevice.h

@@ -99,6 +99,12 @@ public:
      */
     void teardown();
 
+    /* See AP_HAL::SPIDeviceManager::get_count() */
+    uint8_t get_count();
+
+    /* See AP_HAL::SPIDeviceManager::get_device_name() */
+    const char *get_device_name(uint8_t idx);
+
 protected:
     void _unregister(SPIBus &b);
     AP_HAL::OwnPtr<AP_HAL::SPIDevice> _create_device(SPIBus &b, SPIDesc &device_desc) const;