MAINTAINERS: Add Ethernet PHY YAML file

While the Ethernet PHY framework was marked as maintained, the device tree
bindings associated to that framework was not listed under the maintained
files. Fix that.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>