dt-bindings: clock: Document the parent clocks

dt-bindings: clock: Document the parent clocks

With clock parent data scheme we must specify the parent clocks for the
rpmhcc nodes. So describe the parent clock for rpmhcc in the bindings.

Signed-off-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lkml.kernel.org/r/20190826173120.2971-2-vkoul@kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>