dt-binding: dwmac-sun8i: add H6 compatible string (w/ A64 fallback)

dt-binding: dwmac-sun8i: add H6 compatible string (w/ A64 fallback)

The Allwinner H6 SoC features a Ethernet MAC that is similar to the one
in A64.

Add a compatible string for it with A64 fallback compatible string, in
this case the A64 driver can be used.

The "internal" PHY is not internal from the perspective of the H6 main
die, instead it's on the co-packaged AC200 chip, and connected to the
main die with RMII at the in-package Port A PIO bank. So from the SoC
driver side it needs no special treatment.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>