ARM: pmu: remove unused reservation mechanism

The PMU reservation mechanism was originally intended to allow OProfile
and perf-events to co-ordinate over access to the CPU PMU. Since then,
OProfile for ARM has moved to using perf as its backend, so the
reservation code is no longer used.

This patch removes the reservation code for the CPU PMU on ARM.

Signed-off-by: Will Deacon <will.deacon@arm.com>