dt-bindings: net: convert emac_rockchip.txt to YAML

Convert emac_rockchip.txt to YAML.

Changes against original bindings:
  Add mdio sub node.
  Add extra clock for rk3036

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220603163539.537-1-jbx6244@gmail.com