perf/x86: Export some PMU attributes in caps/ directory

perf/x86: Export some PMU attributes in caps/ directory

It can be difficult to figure out for user programs what features
the x86 CPU PMU driver actually supports. Currently it requires
grepping in dmesg, but dmesg is not always available.

This adds a caps directory to /sys/bus/event_source/devices/cpu/,
similar to the caps already used on intel_pt, which can be used to
discover the available capabilities cleanly.

Three capabilities are defined:

 - pmu_name:	Underlying CPU name known to the driver
 - max_precise:	Max precise level supported
 - branches:	Known depth of LBR.

Example:

  % grep . /sys/bus/event_source/devices/cpu/caps/*
  /sys/bus/event_source/devices/cpu/caps/branches:32
  /sys/bus/event_source/devices/cpu/caps/max_precise:3
  /sys/bus/event_source/devices/cpu/caps/pmu_name:skylake

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170822185201.9261-3-andi@firstfloor.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>