dt-bindings: arm: Explain capacities-dmips-mhz calculations in example

The example contains two values for the capacity currently, 446 in text
and 578 in code. The numbers are all correct but can confuse some of the
readers. This patch tries to explain how the numbers are calculated to
avoid same confusion going forward.

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>