qcom: clock-pll: Add support for a variable rate PLL

Some clocks require their PLLs to be reprogrammed to
support various frequency plans. Add support for a PLL
that can change its rate at runtime. It is assumed that
the PLL can only change its rate when the PLL outputs
are off.

Change-Id: I846a53e93afabb1a5f2cb4022f21777456afeb51
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>