clk: qcom: Elaborate on "active" clocks in the RPM clock bindings

The concept of "active" clocks is just explained in a bried comment in the
device driver, let's explain it a bit more in the device tree bindings
so everyone understands this.

Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>