arm64: dts: agilex: add board compatible for N5X DK

The Intel SoCFPGA N5X SoC Development Kit is a board with
Agilex, so it needs its own compatible.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>