fpga manager: xilinx-spi: check INIT_B pin during write_init

The INIT_B pin reports the status during startup and after the end of the
programming process. However the current driver completely ignores it.

Check the pin status during startup to make sure programming is never
started too early and also to detect any hardware issues in the FPGA
connection.

This is optional for backward compatibility. If INIT_B is not passed by
device tree, just fallback to the old udelays.

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