dts: socfpga: Add sysmgr node so the gmac can use to reference

commit[7e0b4cd dts: socfpga: Add DTS entry for adding the stmmac glue
layer for stmmac.] references the sysmgr through its phandle. This patch
adds the appropriate sysmgr node for the gmac to use.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>

arch/arm/boot/dts/socfpga.dtsi

@@ -563,7 +563,7 @@
 			reg = <0xffd05000 0x1000>;
 		};
 
-		sysmgr@ffd08000 {
+		sysmgr: sysmgr@ffd08000 {
 			compatible = "altr,sys-mgr", "syscon";
 			reg = <0xffd08000 0x4000>;
 		};