lisa.analysis.latency: Include CPUs in latency DFs

FEATURE

Latency-related functions such as df_latency_wakeup and
df_latency_preemption at the moment do not return any information about
which CPUs the latency ocurred on.

Add a 'cpu' column to df_latency_preemption along with 'cpu' &
'target_cpu' columns to df_latency_wakeup.

Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>