The "soc" node is supposed to have only MMIO children, so move the FPGA
region node to top level to fix dtc W=1 warnings like:

  socfpga_stratix10.dtsi:136.20-141.5: Warning (simple_bus_reg): /soc@0/base_fpga_region: missing or empty reg/ranges property
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>