ARM: dts: uniphier: Change phy-mode to RGMII-ID to enable delay pins

UniPhier LD20, PXs2 and PXs3 boards have ethernet phy that has RX/TX delays
of RGMII interface using pull-ups on the RXDLY and TXDLY pins.

So should set the phy-mode to "rgmii-id" to show that RX/TX delays are
enabled.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>