dt-bindings: net: phy: gpy2xx: more precise description

Mention that the interrupt line is just asserted for a random period of
time, not the entire time.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>