dt-bindings: Convert usb-connector to YAML format.

Convert the usb-connector.txt bindings file to YAML format. This allows
it to be used in dt_bindings_check verification. This patch was
born out of a patch series for the addition of a Type C connector
class port driver[1].

An attempt has been made to maintain the same documentation text and
example structure as was in the .txt file, but wherever needed
modifications have been made to satisfy dt_bindings_check.

Also, update all references to usb-connector.txt to now use
usb-connector.yaml.

[1]: https://lkml.org/lkml/2020/2/19/1232



Signed-off-by: Prashant Malani <pmalani@chromium.org>
Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Rob Herring <robh@kernel.org>