arm64: dts: uniphier: stabilize ethernet of LD20 reference board

Currently, the ethernet RGMII mode on the LD20 reference board is
unstable.

The default drive-strength of ethernet TX pins is too strong because
there is no dumping resistor on the TX lines on the board.

Weaken the drive-strength to make the ethernet more stable.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>