dev: implement generic devices

generic devices don't have a type (char, block, etc) and are created
by bus drivers to represent the device until a suitable driver is found.

generic devices are registered by the bus driver. once a more suitable
driver is found, that driver will re-register the device under itself.

dev/block.c

@@ -11,3 +11,9 @@ struct block_device *block_device_create(void)
 
 	return BLOCK_DEVICE(dev);
 }
+
+struct block_device *block_device_from_generic(struct device *dev)
+{
+	dev->dev_type = DEV_TYPE_BLOCK;
+	return BLOCK_DEVICE(dev);
+}