dt-bindings: net: snps,dwmac: Document stmmac-axi-config subnode

dt-bindings: net: snps,dwmac: Document stmmac-axi-config subnode

The stmmac-axi-config subnode is present in multiple dwmac instance DTs,
document its content per snps,axi-config property description which is
a phandle to this subnode.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220927012449.698915-1-marex@denx.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>