media: Documentation: PHY information can be obtained from OF endpoint too

bus-type endpoint property contains the information.

Signed-off-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>