arm64: dts: renesas: r8a779a0: Add Ethernet-AVB support

Define the generic parts of Ethernet-AVB device nodes. Only AVB0 was
tested because it was the only port with a PHY on current hardware.
Signed-off-by: Tho Vu <tho.vu.wh@renesas.com>
[wsa: double checked, rebased, added "internal-delay" properties]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20210121100619.5653-4-wsa+renesas@sang-engineering.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>