staging: axis-fifo: Add elaborate description in Kconfig

  - The Xilinx AXI-Stream FIFO IP core driver description is elaborated.
  - References: Xilinx PG080 document, axis-fifo.txt

Signed-off-by: Moses Christopher <moseschristopherb@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>