drivers: clk: st: PLL rate change implementation for DVFS

Change A9 PLL rate, as per requirement from the cpufreq framework,
for DVFS. For rate change, the A9 clock needs to be temporarily sourced
from PLL external to A9 and then sourced back to A9-PLL

Signed-off-by: Pankaj Dev <pankaj.dev@st.com>
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>