[media] doc-rst: add pxa_camera documentation

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

Documentation/media/v4l-drivers/index.rst

@@ -31,4 +31,5 @@ License".
 	omap3isp
 	omap4_camera
 	pvrusb2
+	pxa_camera
 	zr364xx