media: v4l2-fwnode: link_frequency is an optional property

v4l2_fwnode_endpoint_alloc_parse() is intended as a replacement for
v4l2_fwnode_endpoint_parse(). It parses the "link-frequency" property and
if the property isn't found, it returns an error. However,
"link-frequency" is an optional property and if it does not exist is not
an error. Instead, the number of link frequencies is simply zero in that
case.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>