qcom_mdio: add initial IPQ4018 MDIO support

This adds support for the IPQ4018/IPQ4019 MDIO bus.  This is used to
talk to external PHYs and switches.  (There's an internal switch
in the IPQ4018/IPQ4019 as well, but it's accessible via MMIO/AXI.)

Differential Revision: https://reviews.freebsd.org/D34110
Reviewed by: manu

sys/arm/conf/std.qca

@@ -72,3 +72,9 @@ device 		xhci
 device 		dwc3
 device 		qcom_dwc3
 options		USB_HOST_ALIGN=64
+
+# Ethernet support
+device 		mdio
+device 		mii
+device 		miibus
+device 		qcom_mdio_ipq4018

sys/arm/qualcomm/std.ipq4018

@@ -32,3 +32,5 @@ dev/qcom_tlmm/qcom_tlmm_pin.c		optional qcom_tlmm_ipq4018
 dev/qcom_tlmm/qcom_tlmm_pinmux.c	optional qcom_tlmm_ipq4018
 
 dev/qcom_tcsr/qcom_tcsr.c		optional qcom_tcsr
+
+dev/qcom_mdio/qcom_mdio_ipq4018.c	optional qcom_mdio_ipq4018