qcom: fix panic in qcom_mdio_ipq4018 implementation

Fix a boot-time panic in qcom_mdio_ipq4018 due to a missing bus function
and hook the file up to the build so that it will not rot away.

Test booted on an ipq807x in 2022.
X-Differential Revision: extracted from D37882

sys/arm64/conf/std.qcom

@@ -4,6 +4,7 @@
 
 # Qualcomm Snapdragon drivers
 device		qcom_gcc		# Global Clock Controller
+device		qcom_mdio		# MDIO support
 
 # Serial (COM) ports
 device		uart_msm		# Qualcomm MSM UART driver

sys/conf/files.arm64

@@ -643,6 +643,7 @@ dev/flash/flexspi/flex_spi.c    		optional clk flex_spi soc_nxp_ls fdt
 
 # Qualcomm
 arm64/qualcomm/qcom_gcc.c			optional qcom_gcc fdt
+dev/qcom_mdio/qcom_mdio_ipq4018.c		optional qcom_mdio fdt mdio mii
 
 # RockChip Drivers
 arm64/rockchip/rk3328_codec.c			optional fdt rk3328codec soc_rockchip_rk3328

sys/dev/qcom_mdio/qcom_mdio_ipq4018.c

@@ -273,6 +273,9 @@ static device_method_t qcom_mdio_ipq4018_methods[] = {
 	DEVMETHOD(device_attach, qcom_mdio_ipq4018_attach),
 	DEVMETHOD(device_detach, qcom_mdio_ipq4018_detach),
 
+	/* Bus interface */
+	DEVMETHOD(bus_add_child, bus_generic_add_child),
+
 	/* MDIO interface */
 	DEVMETHOD(mdio_readreg, qcom_mdio_ipq4018_readreg),
 	DEVMETHOD(mdio_writereg, qcom_mdio_ipq4018_writereg),