[media] doc-rst: add documentation for si476x

Convert it to ReST and add to media/v4l-drivers book.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/v4l-drivers/index.rst

@@ -36,4 +36,6 @@ License".
 	saa7134
 	sh_mobile_ceu_camera
 	si470x
+	si4713
+	si476x
 	zr364xx