dt-bindings: Merge gpio-usb-b-connector with usb-connector

The binding for usb-connector is a superset of gpio-usb-b-connector. One
major difference is that gpio-usb-b-connector requires at least one of
the vbus-gpios and id-gpios properties to be specified. Merge the two
bindings by adding the compatible string combination for the GPIO USB-B
variant and an extra conditional for the required properties list to the
usb-connector.yaml file.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>