net: mdio-uclass: name MDIO according to device-name property if preset

Use the optional property device-name to name the MDIO bus.  This works
around limitations with using the DT node name on devices such as
Armada-8040, which integrates two cp100 cores, both featuring MDIOs at the
same relative offsets and with the same DT node names.
The concept was originally proposed by Marvell as a custom property called
mdio-name specific to Marvell driver.  This patch uses the more generic
property device-name and moves this into MDIO class code so other can use
it as well.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>