arm64: dts: Add MDIO multiplexer DT node for Stingray

Added MDIO multiplexer iproc DT node for Stingray, which contains
the child nodes of PCIe serdes, RGMII, SATA and USB phy MDIO slaves.

Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>