soc: qcom: add l2 cache perf events driver

Adds perf events support for L2 cache PMU.

The L2 cache PMU driver is named 'l2cache' and can be used
with perf events to profile L2 events such as cache hits
and misses.

Change-Id: Ic17087dfb48781fec8daf0063e5d556d377dbc7e
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>