dt-bindings/perf: Add Arm NI-700 PMU

Add an initial binding for the Arm NI-700 interconnect PMU. As with the
Arm CMN family, there are already future NI products on the roadmap, so
the overall binding is named generically just in case any
non-discoverable incompatibility between generations crops up.

Cc: <devicetree@vger.kernel.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>