dt-bindings: dwc3-xilinx: Add missing comma in example

Trivial example fix.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/8fa5edcaa6b93859cfda97d080aad378e89c1b44.1611232967.git.michal.simek@xilinx.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>