net: stmmac: support future possible different internal phy mode

The current way to find if the phy is internal is to compare DT phy-mode
and emac_variant/internal_phy.
But it will negate a possible future SoC where an external PHY use the
same phy mode than the internal one.

By using phy-mode = "internal" we permit to have an external PHY with
the same mode than the internal one.

Reported-by: André Przywara <andre.przywara@arm.com>
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>