ARM: 5902/4: arm/perfevents: implement perf event support for ARMv6

This patch implements support for ARMv6 performance counters in the
Linux performance events subsystem. ARMv6 architectures that have the
performance counters should enable HW_PERF_EVENTS to get hardware
performance events support in addition to the software events.

Note: only ARM Ltd ARM cores are supported.

This implementation also provides an ARM PMU abstraction layer to allow
ARMv7 and others to be supported in the future by adding new a
'struct arm_pmu'.

Cc: Jean Pihet <jpihet@mvista.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Jamie Iles <jamie.iles@picochip.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>