net: stmmac: Deprecate snps, phy-addr and auto-detect PHY address

The snps,phy-addr device tree property is non-standard, and should be
removed in favor of proper phy node support. Remove it from the binding
documents and warn if the property is still used.

Most PHYs respond to address 0, but a few don't, so auto-detect PHY
address by default, to make up for the lack of explicit address selection.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>