net: dsa: allow drivers to get the port OF node

In the current DSA switch driver API, only the udevice of the switch
(belonging to UCLASS_DSA) is exposed, as well as an "int port" argument.
So drivers do not have access to the udevice of individual ports
(belonging to UCLASS_ETH), one of the reasons being that not all ports
have an associated UCLASS_ETH udevice.

However, all DSA ports have an OF node, and in some cases the driver
needs a handle to it, for all ports including the CPU port. Example: the
following Linux per-port device tree property:

	managed = "in-band-status";

states whether a port should operate with clause 37 in-band autoneg
enabled or not.

This patch exposes a function which can be called by individual drivers
as needed.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>