Perf: Add legacy support for userspace tools

Perf used the "rs" and "rm" modifiers to access
the L2CC and busmon PMU's on legacy systems.

This modifies the parser to keep the backward compatibility
alive till all tools progress to the new named PMU interfaces.

Change-Id: I50dde4252ac949e9cbab7c62536c5fd49177896c
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>