msm: cpp: Use the src clock node to read the cpp src clock rate

Use the src clock binding to read the cpp src clock rate intead of
using the list rate api from clock driver.

CRs-Fixed: 1079648
Change-Id: I2ebdc0c41c521e56a400d685b6254174cffdc14b
Signed-off-by: Krishnankutty Kolathappilly <kkolatha@codeaurora.org>