ARM: perf: encode PMU name in arm_pmu structure

Currently, perf uses the PMU ID as an index into a string table
to look up the name of a given PMU.

This patch encodes the name of a PMU directly into the arm_pmu
structure so that PMU-specific code can be factored out into
separate files.

Acked-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>