msm: isp: Get clock rates on run time

ISP decides to run on single VFE or dual VFE
based on the rate of pixels received, compared
with VFE's clock rate.

This change retrieves nominal and turbo clock rates of vfe
in runtime.

Change-Id: I57cd64382a01488a5850f5f289ebdc5007b3627c
Signed-off-by: Senthil Kumar Rajagopal <skrajago@codeaurora.org>
Signed-off-by: Sreesudhan Ramakrish Ramkumar <srramku@codeaurora.org>