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

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

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

Change-Id: I4a78f253d0449c2ad4fb00ead84be71d0c023a8d
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>