ar40xx_switch: add initial switch for the IPQ4018/IPQ4019.

Summary:

This switch is based off of the AR8327/AR8337 external switch/PHY.
However unlike the AR8327/AR8337 it itself doesn't have any PHYs;
instead an external PHY connects to it using the PSGMII port.

Differential Revision: https://reviews.freebsd.org/D34112
Reviewed by: manu

This code is inspired by the ar40xx code in openwrt, which itself
is based on the Qualcomm QCA-SSDK.  Both of these sources are, amusingly,
BSD licenced - and thus I have included some of the comments in the
hardware workaround paths to document some of the magic numbers.

sys/arm/conf/std.qca

@@ -78,3 +78,5 @@ device 		mdio
 device 		mii
 device 		miibus
 device 		qcom_mdio_ipq4018
+device 		etherswitch
+device 		ar40xx_switch

sys/arm/qualcomm/std.ipq4018

@@ -6,6 +6,27 @@ arm/qualcomm/qcom_cpu_kpssv2.c		optional smp
 arm/qualcomm/ipq4018_usb_hs_phy.c	optional qcom_ipq4018_hs_usbphy
 arm/qualcomm/ipq4018_usb_ss_phy.c	optional qcom_ipq4018_ss_usbphy
 
+dev/etherswitch/ar40xx/ar40xx_main.c	\
+		optional mdio etherswitch ar40xx_switch
+dev/etherswitch/ar40xx/ar40xx_phy.c	\
+		optional mdio etherswitch ar40xx_switch
+dev/etherswitch/ar40xx/ar40xx_hw.c	\
+		optional mdio etherswitch ar40xx_switch
+dev/etherswitch/ar40xx/ar40xx_hw_atu.c	\
+		optional mdio etherswitch ar40xx_switch
+dev/etherswitch/ar40xx/ar40xx_hw_port.c	\
+		optional mdio etherswitch ar40xx_switch
+dev/etherswitch/ar40xx/ar40xx_hw_psgmii.c	\
+		optional mdio etherswitch ar40xx_switch
+dev/etherswitch/ar40xx/ar40xx_hw_mib.c	\
+		optional mdio etherswitch ar40xx_switch
+dev/etherswitch/ar40xx/ar40xx_hw_mirror.c	\
+		optional mdio etherswitch ar40xx_switch
+dev/etherswitch/ar40xx/ar40xx_hw_vtu.c	\
+		optional mdio etherswitch ar40xx_switch
+dev/etherswitch/ar40xx/ar40xx_hw_mdio.c	\
+		optional mdio etherswitch ar40xx_switch
+
 dev/qcom_dwc3/qcom_dwc3.c		optional qcom_dwc3
 
 dev/qcom_rnd/qcom_rnd.c			optional qcom_rnd