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.