interconnect: Sanity check that node isn't already on list

Broken interconnect providers might add the same node multiple times or
in multiple providers, which causes strange errors as the provider's
node list is later traversed.

Detect that a node already has an associated provider, complain and
reject the addition of the node, to aid the developer.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210625234903.1324755-1-bjorn.andersson@linaro.org


Signed-off-by: Georgi Djakov <djakov@kernel.org>