doc: bindings: Add description for MDIO MUX dts nodes

Adds a short bindings document describing the expected structure of a MDIO
MUX dts node.  This is based on Linux binding and the example is in fact
copied from there.

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>