net: stmmac: Make MDIO bus reset optional

The Tegra EQOS driver already resets the MDIO bus at probe time via the
reset GPIO specified in the phy-reset-gpios device tree property. There
is no need to reset the bus again later on.

This avoids the need to query the device tree for the snps,reset GPIO,
which is not part of the Tegra EQOS device tree bindings. This quiesces
an error message from the generic bus reset code if it doesn't find the
snps,reset related delays.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>