of: Check for overlap in reserved memory regions

Any overlap in the reserved memory regions (those specified in the
reserved-memory DT node) is a bug.  These bugs might go undetected as
long as the contested region isn't used simultaneously by multiple
software agents, which makes such bugs hard to debug.  Fix this by
printing a scary warning during boot if overlap is detected.

Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Rob Herring <robh@kernel.org>