arm64: dts: socfpga: agilex: Fix gmac compatible

Fix gmac compatible string to "altr,socfpga-stmmac-a10-s10". Gmac for
Agilex should use same compatible as Stratix 10.

Fixes: 4b36daf9 ("arm64: dts: agilex: Add initial support for Intel's Agilex SoCFPGA")
Cc: stable@vger.kernel.org
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>