arm64: dts: stratix10: add board compatible for SoCFPGA DK

The Altera SoCFPGA Stratix 10 SoC Development Kit is a board with
Stratix 10, so it needs its own compatible.

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