To enable sharing of the arm_pmu code with arm64, this patch factors it
out to drivers/perf/. A new drivers/perf directory is added for
performance monitor drivers to live under.

MAINTAINERS is updated accordingly. Files added previously without a
corresponsing MAINTAINERS update (perf_regs.c, perf_callchain.c, and
perf_event.h) are also added.

Cc: Arnaldo Carvalho de Melo
Cc: Greg Kroah-Hartman
Cc: Ingo Molnar
Cc: Linus Walleij
Cc: Paul Mackerras
Cc: Peter Zijlstra
Cc: Russell King
Cc: Will Deacon
Signed-off-by: Mark Rutland
[will: augmented Kconfig help slightly]
Signed-off-by: Will Deacon