dt-bindings: usb: document role-switch-default-mode property in dwc2

role-switch-default-mode property may be used with usb-role-switch
property to define the default operation mode (by forcing the a-valid or
b-valid) at init.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/1638806203-6624-2-git-send-email-fabrice.gasnier@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>