net: stmmac: dwmac-qcom-ethqos: Adjust rgmii loopback_en per platform

Not all platforms should have RGMII_CONFIG_LOOPBACK_EN and the result it
about 50% packet loss on incoming messages. So make it possile to
configure this per compatible and enable it for QCS404.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>