Documentation: Add binding document for Lattice iCE40 FPGA manager

This adds documentation of the device tree bindings of the Lattice iCE40
FPGA driver for the FPGA manager framework.

Signed-off-by: Joel Holdsworth <joel@airwebreathe.org.uk>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Alan Tull <atull@opensource.altera.com>
Acked-by: Moritz Fischer <moritz.fischer@ettus.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>