riscv: dts: thead: Add TH1520 ethernet nodes

Add gmac, mdio, and phy nodes to enable the gigabit Ethernet ports on
the BeagleV Ahead and Sipeed Lichee Pi 4a boards.

Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
[drew: change apb registers from syscon to second reg of gmac node,
       add phy reset delay properties for beaglev ahead]
Signed-off-by: Drew Fustini <dfustini@tenstorrent.com>