fpga: region: add missing DT documentation for config complete timeout

Commit 42d5ec95 ("fpga: add config complete timeout") introduced the
config complete property but didn't include the corresponding DT binding
documentation. Add it now.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Reviewed-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>