docs: misc: xilinx_sdfec: Actually add documentation

Add SD-FEC driver documentation.
Signed-off-by: Derek Kiernan <derek.kiernan@xilinx.com>
Signed-off-by: Dragan Cvetic <dragan.cvetic@xilinx.com>
Link: https://lore.kernel.org/r/1560274185-264438-11-git-send-email-dragan.cvetic@xilinx.com
[kees: extracted from v7 as it was missing in the commit for v8]
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>