dt-bindings: net: mdio: add reset-post-delay-us property

Add "reset-post-delay-us" parameter to MDIO bus properties,
so it's possible to add a delay after reset deassert.
This is optional in case external hardware slows down
release of the reset signal.

Signed-off-by: Bruno Thomsen <bruno.thomsen@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>