dt-bindings: net: micrel: add coma-mode-gpios property

The LAN8814 has a coma mode pin which is used to put the PHY into
isolate and power-down mode. Usually strapped to be asserted by default.
A GPIO is then used to take the PHY out of this mode.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>