docs: driver-api: add xilinx driver API documentation

The current file there (emmi) provides a description of
the driver uAPI and kAPI.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/driver-api/index.rst

@@ -93,6 +93,7 @@ available subsections can be seen below.
    sync_file
    vfio-mediated-device
    vfio
+   xilinx/index
    xillybus
    zorro
 

Documentation/xilinx/index.rst → Documentation/driver-api/xilinx/index.rst

-:orphan:
 
 ===========
 Xilinx FPGA