clk: qcom: Configure the RCGs to a safe source as needed

For some root clock generators, there could be child branches which are
controlled by an entity other than application processor subsystem. For
such RCGs, as per application processor subsystem clock driver, all of its
downstream clocks are disabled and RCG is in disabled state but in actual
downstream clocks can be left enabled before.

So in this scenario, when RCG is disabled as per clock driver's point of
view and when rate scaling request comes before downstream clock enable
request, then RCG fails to update its configuration because in actual RCG
is on and it expects its new source to alredy in enable state  but in
reality new source is in off state. In order to avoid letting the RCG to
go into an invalid state, add support to just cache the rate of RCG during
set_rate(), defer actual RCG configuration update to be done during
clk_enable() as at this point of time, both its new parent and safe source
will be already enabled and RCG can safely switch to new parent.

During clk_disable() request, configure it to safe source as both
its parents, safe source and current parent will be enabled and RCG can
safely execute a switch. Also add support to have safe configuration
frequency table structure for each shared RCG.

Signed-off-by: Taniya Das <tdas@codeaurora.org>
Signed-off-by: Amit Nischal <anischal@codeaurora.org>