arm64: dts: renesas: ulcb: Override EtherAVB phy-mode

As the PHY interface installed on the ULCB board provides TX
channel delay, make the "phy-mode" property a board-specific one, meant
to override the one specified in the SoC DTSI.

Follow up patches will reset the r8a7795/96 SoC DTSI to use "rgmii" mode\
and let the board files override that.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>