dt: bindings: fpga: add lattice machxo2 slave spi binding description

Add dt binding documentation details for Lattice MachXO2 FPGA configuration
over Slave SPI interface.

Signed-off-by: Paolo Pisati <p.pisati@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>