of: property: fw_devlink: Add support for "phy-handle" property

Allows tracking dependencies between Ethernet PHYs and their consumers.

Cc: Andrew Lunn <andrew@lunn.ch>
Cc: netdev@vger.kernel.org
Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20210818021717.3268255-1-saravanak@google.com
Signed-off-by: Rob Herring <robh@kernel.org>