ARM: dts: arria10: add board compatible for SoCFPGA DK

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

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