arm64: dts: renesas: cat875: Drop superfluous phy-mode

The PHY mode already defaults to RGMII in the RZ/G2E base SoC DTS file,
so there is no need to specify the same value in board files.

Fixes: 6b170cd3 ("arm64: dts: renesas: cat875: Add ethernet support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20200706151400.23105-1-geert+renesas@glider.be