"When using crystal, clock is generated internally after the power is
stable. In order to get reliable power-on-reset, it is recommended to
keep asserting the reset low signal long enough (10 ms) to ensure the
clock is stable and clock-to-reset (1 ms) requirement is satisfied."
Pass the 'reset-assert-us' property to describe such requirement.
While at it, use the 'reset-gpios' property inside the the mdio
node instead of the deprecated usage of 'phy-reset-gpios'.