PM / devfreq: memlat: simplify core-dev table parsing logic

In register_common(), there's unnecessary code used to find
which device tree node has the core-dev property. Refactor
this to reduce complexity.

Change-Id: Ib3475272b25e898ad23f9f5a4412d90cd889a356
Signed-off-by: Amir Vajid <avajid@codeaurora.org>