documentation: fpga: add bridge document to driver-api

Add a new document to driver-api/fpga that documents the
fpga bridge API and add it to driver-api/fpga/index.rst

Signed-off-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/fpga/index.rst

@@ -9,3 +9,4 @@ FPGA Subsystem
 
    intro
    fpga-mgr
+   fpga-bridge