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

commit[7e0b4cd0 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>