clk: qcom: Add support for RCGs with shared branches

Some root clock generators may have child branches that are controlled
by different CPUs. These RCGs require some special operations:
 - some enable bits have to be toggled when we set the rate;
 - if RCG is disabled we only cache the rate and set it later when enabled;
 - when the RCG is disabled, the mux is set to the safe source;

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>
[sboyd@codeaurora.org: Simplify recalc_rate implementation]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>