bindings: net: stmmac: correct note about TSO

snps,tso was previously placed under AXI BUS Mode parameters,
suggesting that the property should be in the stmmac-axi-config node.

TSO (TCP Segmentation Offloading) has nothing to do with AXI BUS Mode
parameters, and the parser actually expects it to be in the root node,
not in the stmmac-axi-config.

Also added a note about snps,tso only being available on GMAC4 and newer.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>