arm: dts: dra7: add syscon phandle to cpsw node

There are 2 MACIDs stored in the control module of the dra7.
These are read by the cpsw driver if no valid MACID was found
in the devicetree.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>