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: NTobias Klauser <tklauser@distanz.ch>
Reviewed-by: NMoritz Fischer <mdf@kernel.org>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/fpga/fpga-region.txt

@@ -192,6 +192,8 @@ Optional properties:
 - region-freeze-timeout-us : The maximum time in microseconds to wait for
 	bridges to successfully become disabled before the region has been
 	programmed.
+- config-complete-timeout-us : The maximum time in microseconds time for the
+	FPGA to go to operating mode after the region has been programmed.
 - child nodes : devices in the FPGA after programming.
 
 In the example below, when an overlay is applied targeting fpga-region0,