dt-bindings: csky: Add csky PMU bindings

This patch adds the documentation to describe that how to add pmu node in
dts.

Signed-off-by: Mao Han <han_mao@c-sky.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>