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