Perf: Add named PMU support for Scorpion and Krait

From v3.4 onwards PMU's can be accessed using names
listed under /sys/bus/event_source/devices/

This patch sets up format attributes per PMU to describe how to
construct events for each PMU.

Change-Id: I2dd1293e0a906c3075c02a125ccceefeb33a80e3
Signed-off-by: Ashwin Chaugule <ashwinc@codeaurora.org>