add sysfs document for fpga bridge class

Add documentation for new FPGA bridge class's sysfs interface.

Signed-off-by: Alan Tull <atull@opensource.altera.com>
Acked-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>