arm64: dts: stratix10: add the sysmgr-syscon property from the gmac's

The gmac ethernet driver uses the "altr,sysmgr-syscon" property to
configure phy settings for the gmac controller.

Add the "altr,sysmgr-syscon" property to all gmac nodes.

This patch fixes:

[    0.917530] socfpga-dwmac ff800000.ethernet: No sysmgr-syscon node found
[    0.924209] socfpga-dwmac ff800000.ethernet: Unable to parse OF data

Cc: stable@vger.kernel.org
Reported-by: Ley Foon Tan <ley.foon.tan@intel.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>