of: property: add of_graph_get_next_port_endpoint()

We already have of_graph_get_next_endpoint(), but it is not
intuitive to use in some case.

(X) node {
(Y) ports {
(P0) port@0 { endpoint { remote-endpoint = ...; };};
(P10) port@1 { endpoint { remote-endpoint = ...; };
(P11) endpoint { remote-endpoint = ...; };};
(P2) port@2 { endpoint { remote-endpoint = ...; };};
};
};

For example, if I want to handle port@1's 2 endpoints (= P10, P11),
I want to use like below

P10 = of_graph_get_next_endpoint(port1, NULL);
P11 = of_graph_get_next_endpoint(port1, P10);

But 1st one will be error, because of_graph_get_next_endpoint()
requested 1st parameter is "node" (X) or "ports" (Y), not but "port".
Below works well, but it will get P0

P0 = of_graph_get_next_endpoint(node, NULL);
P0 = of_graph_get_next_endpoint(ports, NULL);

In other words, we can't handle P10/P11 directly via
of_graph_get_next_endpoint().

There is another non intuitive behavior on of_graph_get_next_endpoint().
In case of if I could get P10 pointer for some way, and if I want to
handle port@1 things by loop, I would like use it like below

/*
* "ep" is now P10, and handle port1 things here,
* but we don't know how many endpoints port1 have.
*
* Because "ep" is non NULL now, we can use port1
* as of_graph_get_next_endpoint(port1, xxx)
*/
do {
/* do something for port1 specific things here */
} while (ep = of_graph_get_next_endpoint(port1, ep))

But it also not worked as I expected.
I expect it will be P10 -> P11 -> NULL,
but it will be P10 -> P11 -> P2, because
of_graph_get_next_endpoint() will fetch "endpoint" beyond the "port".
It is not useful for generic driver.

To handle endpoint more intuitive, create of_graph_get_next_port_endpoint()

of_graph_get_next_port_endpoint(port1, NULL); // P10
of_graph_get_next_port_endpoint(port1, P10); // P11
of_graph_get_next_port_endpoint(port1, P11); // NULL

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87jzdyb5t5.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>