ARM: dts: aspeed: elbert: Enable mac3 controller

Enable mac3 controller in Elbert dts: Elbert MAC3 is connected to the
BCM53134P onboard switch's IMP_RGMII port directly (fixed link, no PHY
between BMC MAC and BCM53134P).

Note: BMC's mdio0 controller is connected to BCM53134P's MDIO interface,
and the MDIO channel will be enabled later, when BCM53134 is added to
"bcm53xx" DSA driver.

Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>