ARM: perf: use integers for ARMv7 event indices

This patch ensures that integers are used to represent event indices in
the ARMv7 PMU backend. This ensures consistency between functions and
also with the arm_pmu structure.

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