msm: dcvs: split get_core in add/get_core

There is currently only one function to add and get a core. Clean this
up by splitting that in two functions one for adding core and the other
for getting one.

Change-Id: Ia9d0b9aa70328fe355ea020f68100c38f2129c2d
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>