ARM: dts: sunxi: Switch to the generic PHY properties

The DWMAC specific properties to manage the PHY have been superseeded by
the generic PHY properties. Let's move to it.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>