MIPS: dts: ocelot: describe the load/save GPIO

This patch adds a description of the load/save GPIN pin, used in the
VSC8584 PHY for timestamping operations. The related pinctrl description
is also added.

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