dt-bindings: fpga: document Lattice sysCONFIG FPGA manager

Add Device Tree Binding doc for configuring Lattice ECP5 FPGA over
Slave SPI sysCONFIG interface.

Signed-off-by: Ivan Bornyakov <i.bornyakov@metrotek.ru>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20221025053947.2737-3-i.bornyakov@metrotek.ru


Signed-off-by: Xu Yilun <yilun.xu@intel.com>