ARM: dts: at91: tse850: the emac<->phy interface is rmii

This went unnoticed until commit 7897b071 ("net: macb: convert
to phylink") which tickled the problem. The sama5d3 emac has never
been capable of rgmii, and it all just happened to work before that
commit.

Fixes: 21dd0ece ("ARM: dts: at91: add devicetree for the Axentia TSE-850")
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/ea781f5e-422f-6cbf-3cf4-d5a7bac9392d@axentia.se