ARM: perf: index ARMv7 event counters starting from zero

The current ARMv7 PMU backend indexes event counters from two, with
index zero being reserved and index one being used to represent the
cycle counter.

This patch tidies up the code by indexing from one instead (with zero
for the cycle counter). This allows us to remove many of the accessor
macros along with the counter enumeration and makes the code much more
readable.
Acked-by: Jamie Iles <jamie@jamieiles.com>
Reviewed-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>