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/xilinx/index.rst

@@ -0,0 +1,16 @@
+
+===========
+Xilinx FPGA
+===========
+
+.. toctree::
+    :maxdepth: 1
+
+    eemi
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`