arch: arm64: intel: add dts file to support sgmii ethernet on FPGA side

Based on FPGA hw design that includes sgmii ethernet feature,
add corresponding dts file to enable ethernet port on agilex
board.
Create this dts file refer to below links from Intel SoC PFGA Community
https://releases.rocketboards.org/release/dts-source/agilex/
https://releases.rocketboards.org/release/2021.11/dts-source/agilex/gsrd/

Signed-off-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Liwei Song <liwei.song@windriver.com>