dt-bindings: usb: dwc2: document TPL support

The target peripheral list (TPL) is used to identify targeted devices
during Embedded Host compliance testing. The user can add "tpl-support"
in the device tree to enable it.
Document TPL support as described in usb-hcd.yaml.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20220621152350.145745-4-fabrice.gasnier@foss.st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>