perf: add raw event selection by reg/code/group for scorpion l2

Add the ability to specify events using the register, code, group
format instead of using enumeration values for scorpion L2 cache
raw perf events.

The existing enumeration values are temporarily left in place as they
are used by legacy copies of tools such as QView.

Change-Id: Id3e9fc8e13f61b2a0a811e1cdb458faaf52c5d02
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>