dt-bindings: net: ar803x: add disable-hibernation-mode propetry

The hibernation mode of Atheros AR803x PHYs defaults to be
enabled after hardware reset. When the cable is unplugged,
the PHY will enter hibernation mode after about 10 seconds
and the PHY clocks will be stopped to save power.
However, some MACs need the phy output clock for proper
functioning of their logic. For instance, stmmac needs the
RX_CLK of PHY for software reset to complete.
Therefore, add a DT property to configure the PHY to disable
this hardware hibernation mode.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>