dt-bindings: fpga: xilinx-slave-serial: add optional INIT_B GPIO

The INIT_B is used by the 6 and 7 series to report the programming status,
providing more control and information about programming errors.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Moritz Fischer <mdf@kernel.org>