dts: hisi: fixes no syscon fault when init mdio

dts: hisi: fixes no syscon fault when init mdio

When linux start up, we get the log below:
"Hi-HNS_MDIO 803c0000.mdio: no syscon hisilicon,peri-c-subctrl
mdio_bus mdio@803c0000: mdio sys ctl reg has not maped"

The source code about the subctrl is dealt syscon, but dts doesn't.
It cause such fault, so this patch adds the syscon info on dts files to
fixes it.

Signed-off-by: Kejian Yan <yankejian@huawei.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>