dt-bindings: net: phy: vsc8531: document the load/save GPIO

A new optional property can be used to reference the load/save GPIO,
used for PTP hardware clock (PHC) operations. This patch documents it in
the binding documentation.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>