arm64: dts: allwinner: a64: a64-olinuxino: set the PHY TX delay

The PHY found on the A64-OLinuXino requires a TX delay in order to
operate properly. Olimex uses a 600ps second delay in their BSP, and
that has been found to work, so let's use that value in the current
DT.

Signed-off-by: Rodrigo Exterckötter Tjäder <rodrigo@tjader.xyz>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>