clk: qcom: clk-rcg2: Read RCG source before calculating clk rate

If the clock rate is set previously to hlos and enable
safe config is true. In that case we need to check the
rcg source to make sure recalc will return correct rate.
Rate calculation is done based on selected source rather
than 19.2 MHZ. Also, we are setting correct src value for
XO based on the parent map of a particular clock.

Change-Id: Id6d2923136a0848d55cff37fb5ef2e3b092af27a
Signed-off-by: Shefali Jain <shefjain@codeaurora.org>