net: stmmac: dwmac-starfive: Add phy interface settings

dwmac supports multiple modess. When working under rmii and rgmii,
you need to set different phy interfaces.

According to the dwmac document, when working in rmii, it needs to be
set to 0x4, and rgmii needs to be set to 0x1.

The phy interface needs to be set in syscon, the format is as follows:
starfive,syscon: <&syscon, offset, shift>

Tested-by: Tommaso Merciai <tomm.merciai@gmail.com>
Signed-off-by: Samin Guo <samin.guo@starfivetech.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>