Docs: misc: xilinx_sdfec: 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/1564216438-322406-8-git-send-email-dragan.cvetic@xilinx.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>