arm: rmobile: rzg2_beacon: Enable alternative Ethernet PHY

Due to the part shortage, the AR8031 PHY was replaced with a
Micrel KSZ9131. Enabling both config options keeps backward
compatibility with either platform, and both appear to be
auto-detected.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@gmail.com>