perf: riscv: Preliminary Perf Event Support on RISC-V

This implements the baseline PMU for RISC-V platforms.

To ease future PMU portings, a guide is also written, containing
perf concepts, arch porting practices and some hints.

Changes in v5:
 - Fix patch errors from checkpatch.pl.

Changes in v4:
 - Fix several compilation errors.  Sorry for that.
 - Raise a warning in the write_counter body.

Changes in v3:
 - Fix typos in the document.
 - Change the initialization routine from statically assigning PMU to
   device-tree-based methods, and set default to the PMU proposed in
   this patch.

Changes in v2:
 - Fix the bug reported by Alex, which was caused by not sufficient
   initialization.  Check https://lkml.org/lkml/2018/3/31/251 for the
   discussion.

Alan Kao (2):
  perf: riscv: preliminary RISC-V support
  perf: riscv: Add Document for Future Porting Guide

 Documentation/riscv/pmu.txt         | 249 ++++++++++++++
 arch/riscv/Kconfig                  |  13 +
 arch/riscv/include/asm/perf_event.h |  79 ++++-
 arch/riscv/kernel/Makefile          |   1 +
 arch/riscv/kernel/perf_event.c      | 485 ++++++++++++++++++++++++++++
 5 files changed, 823 insertions(+), 4 deletions(-)
 create mode 100644 Documentation/riscv/pmu.txt
 create mode 100644 arch/riscv/kernel/perf_event.c

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2.17.0