[arm64] net: broadcom - Add UniMAC MDIO module for Genet

The current version of Broadcom's Genet adapter requires UniMAC MDIO to function properly, so add it as an explicit module.

This aims at fixing bug #985956, that prevents the installation of Debian Bullseye on Raspberry Pi 4.

NB: This patch was generated manually, as I was unable to get KConfig to run in my environment.