net: phy: aquantia: add support for aqr115c

Add support for a new model to the Aquantia driver. This PHY supports
2.5 gigabit speeds. The PHY mode is referred to by the manufacturer as
Overclocked SGMII (OCSGMII) but this actually is just 2500BASEX without
in-band signalling so reuse the existing mode to avoid changing the
uAPI.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>