of: property: add of_graph_get_next_port()

We have endpoint base functions
	- of_graph_get_next_endpoint()
	- of_graph_get_endpoint_count()
	- for_each_endpoint_of_node()

Here, for_each_endpoint_of_node() loop finds each endpoints

	ports {
		port@0 {
(1)			endpoint {...};
		};
		port@1 {
(2)			endpoint {...};
		};
		...
	};

In above case, it finds endpoint as (1) -> (2) -> ...

Basically, user/driver knows which port is used for what, but not in
all cases. For example on flexible/generic driver case, how many ports
are used is not fixed.

For example Sound Generic Card driver which is very flexible/generic and
used from many venders can't know how many ports are used, and used for
what, because it depends on each vender SoC and/or its used board.

And more, the port can have multi endpoints. For example Generic Sound
Card case, it supports many type of connection between CPU / Codec, and
some of them uses multi endpoint in one port. see below.

	ports {
(A)		port@0 {
(1)			endpoint@0 {...};
(2)			endpoint@1 {...};
		};
(B)		port@1 {
(3)			endpoint {...};
		};
		...
	};

Generic Sound Card want to handle each connection via "port" base instead
of "endpoint" base. But, it is very difficult to handle each "port" via
existing for_each_endpoint_of_node(). Because getting each "port" via
of_get_parent() from each "endpoint" doesn't work. For example in above
case, both (1) (2) endpoint has same "port" (= A).

Add "port" base functions.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87ldyeb5t9.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

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