of: extra: Introduce ofnode_phy_is_fixed_link() API

of: extra: Introduce ofnode_phy_is_fixed_link() API

Introduce a helper API ofnode_phy_is_fixed_link() to detect whether
the ethernet controller connects to a fixed-link pseudo-PHY device.

Note there are two ways to describe a fixed PHY attached to an
Ethernet device:

- the new DT binding, where 'fixed-link' is a sub-node of the
  Ethernet device
- the old DT binding, where 'fixed-link' is a property with 5
  cells encoding various information about the fixed PHY

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>