qcom_ess_edma: Add the IPQ4018/IPQ4019 ethernet MAC/MDIO driver.

This adds the ESS EDMA driver introduced by the IPQ4018/IPQ4019.

It provides a number of transmit and receive rings which can be mapped
into virtual ethernet devices, which this driver supports.

It's partially integrated into the ar40xx etherswitch which supplies
the port and some filtering/VPN offload functionality. This driver
only currently supports the per-port options which allow for the
virtual ethernet driver mapping.

This was written by reverse engineering the functionality of the
ethernet switch and ethernet driver support provided by Qualcomm
Atheros via their OpenWRT contributions. The code is all originally
authored by myself.

Differential Revision: https://reviews.freebsd.org/D49027