perf tools: add infrastructure for PMU specific configuration

perf tools: add infrastructure for PMU specific configuration

This patchset adds PMU driver specific configuration to the parser
infrastructure by preceding any term with the '@' letter.  As such
doing something like:

perf -e some_event/@drv1,@drv2=drv_config/ ...

will see 'drv1' and 'drv2=config' being added to the list of evsel config
terms.  Token 'drv1' and 'drv2=config' are not processed in user space
and are meant to be interpreted by the PMU driver.

First the lexer/parser are supplemented with the required definitions to
recognise the driver specific configuration.  From there they are simply
added to the list of event terms.  The bulk of the work is done in
function "parse_events_add_pmu()" where driver config event terms are
added to a new list of driver config terms, which in turn spliced with
the event's new driver configuration list.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>