dt-bindings: clock: Document qcom,hfpll

Adds bindings document for qcom,hfpll instantiated within
the Krait processor subsystem as separate register region.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Tested-by: Craig Tatlor <ctatlor97@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>