dt-bindings: perf: Convert apm,xgene-pmu to DT schema

Convert the Applied Micro X-Gene PMU binding to DT schema format. It is
a straightforward conversion.

Link: https://lore.kernel.org/r/20250812181422.68286-1-robh@kernel.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>