perf: qcom_l2_pmu: add event names

Add event names so that common events can be
specified symbolically, for example:

l2cache_0/total-reads/,l2cache_0/cycles/

Event names are displayed in 'perf list'.

Signed-off-by: Neil Leeder <nleeder@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>