arm64: dts: qcom: ipq8074: add MDIO bus

IPQ8074 uses an IPQ4019 compatible MDIO controller that is already
supported in the kernel, so add the DT node in order to use it.

Signed-off-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211007115846.26255-1-robimarko@gmail.com