ARM: dts: sun7i: pcduino3-nano: enable RGMII RX/TX delay on PHY

The RX/TX delays for the Ethernet PHY on the Linksprite pcDuino 3 Nano
are configured in hardware, using resistors that are populated to pull
the RTL8211E's RXDLY/TXDLY pins low or high as needed.

phy-mode should be set to rgmii-id to reflect this. Previously it was
set to rgmii, which used to work but now results in the delays being
disabled again as a result of the bugfix in commit bbc4d71d ("net:
phy: realtek: fix rtl8211e rx/tx delay config").

Tested on two pcDuino 3 Nano boards purchased in 2015. Without this fix,
Ethernet works unreliably on one board and doesn't work at all on the
other.

Fixes: 061035d4 ("ARM: dts: sun7i: Add dts file for pcDuino 3 Nano board")
Signed-off-by: Adam Sampson <ats@offog.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20201123174739.6809-1-ats@offog.org