ARM: dts: sun6i: a31-hummingbird: Enable RGMII RX/TX delay on Ethernet PHY

The Ethernet PHY on the A31 Hummingbird has the RX and TX delays
enabled on the PHY, using pull-ups on the RXDLY and TXDLY pins.

Fix the phy-mode description to correct reflect this so that the
implementation doesn't reconfigure the delays incorrectly. This
happened with commit bbc4d71d ("net: phy: realtek: fix rtl8211e
rx/tx delay config").

Fixes: c220aec2 ("ARM: dts: sun6i: Add Merrii A31 Hummingbird support")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://lore.kernel.org/r/20201024162515.30032-2-wens@kernel.org