clk: msm: clock-cpu-8996: Add support for a droop detection mechanism

The CPU clock subsystem on the MSM8996 V3 has a droop detection
mechanism. Add support for it - this involves some initial
configuration, and using the droop-detection-enabled mux
selection instead of the primary PLL.

This also requires us to switch the GFMUX across DCVS changes
(both voltage and frequency). Furthermore, we disallow L2 GDHS
across DCVS by modifying the pm_qos latency request accordingly.

Change-Id: Icb455437c5251b057032b583ea145733cef5d8bc
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>