clk: qcom: cpu-8996: Add support to switch to alternate PLL

Each of the CPU clusters on msm8996 are powered via a primary
PLL and a secondary PLL. The primary PLL is what drives the
CPU clk, except for times when we are reprogramming the PLL
itself, when we temporarily switch to an alternate PLL.
Use clock rate change notifiers to support this.

Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Ilia Lin <ilialin@codeaurora.org>
Tested-by: Amit Kucheria <amit.kucheria@linaro.org>