doc: bindings: add mdio.txt describing generic MDIO properties

doc: bindings: add mdio.txt describing generic MDIO properties

Adds a binding document for mdio.  A notable deviation from corresponding
Linux binding is the introduction of device-name optional property, which
can be used to name MDIO buses.  Two reset optional properties described
by Linux binding are also not present as they don't seem to be used in
U-Boot at this time.

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>
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>